Fix behavior when merging object,primitive,object.

The AbstractConfigObject.merge() method was wrong; it merged in the
wrong order, which produced incorrect results. But there was a test
verifying that the results were wrong! So it's OK right?
Anyway, fix the test, fix the code, be sure it's crystal clear in spec.

HOCON.md

@@ -150,7 +150,12 @@ To merge objects:
    these same rules.
 
 Object merge can be prevented by setting the key to another value
-first.
+first. This is because merging is always done two values at a
+time; if you set a key to an object, a non-object, then an object,
+first the non-object falls back to the object (non-object always
+wins), and then the object falls back to the non-object (no
+merging, object is the new value). So the two objects never see
+each other.
 
 These two are equivalent:
 
@@ -757,6 +762,48 @@ might wish to support both the SI power of ten units and the IEC
 power of two units. But until an implementation needs that, no
 such thing is documented here.)
 
+### Config object merging and file merging
+
+It may be useful to offer a method to merge two objects. If such a
+method is provided, it should work as if the two objects were
+duplicate values for the same key in the same file. (See the
+section earlier on duplicate key handling.)
+
+As with duplicate keys, an intermediate non-object value "hides"
+earlier object values. So say you merge three objects in this
+order:
+
+ - `{ a : { x : 1 } }`  (first priority)
+ - `{ a : 42 }` (fallback)
+ - `{ a : { y : 2 } }` (another fallback)
+
+The result would be `{ a : { x : 1 } }`. The two objects are not
+merged because they are not "adjacent"; the merging is done in
+pairs, and when `42` is paired with `{ y : 2 }`, `42` simply wins
+and loses all information about what it overrode.
+
+But if you re-ordered like this:
+
+ - `{ a : { x : 1 } }`  (first priority)
+ - `{ a : { y : 2 } }` (fallback)
+ - `{ a : 42 }` (another fallback)
+
+Now the result would be `{ a : { x : 1, y : 2 } }` because the two
+objects are adjacent.
+
+This rule for merging objects loaded from different files is
+_exactly_ the same behavior as for merging duplicate fields in the
+same file. All merging works the same way.
+
+Needless to say, normally it's well-defined whether a config
+setting is supposed to be a number or an object. This kind of
+weird pathology where the two are mixed should not be happening.
+
+The one place where it matters, though, is that it allows you to
+"clear" an object and start over by setting it to null and then
+setting it back to a new object. So this behavior gives people a
+way to get rid of default fallback values they don't want.
+
 ### Java properties mapping
 
 It may be useful to merge Java properties data with data loaded

src/main/java/com/typesafe/config/impl/AbstractConfigObject.java

@@ -7,6 +7,7 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
+import java.util.ListIterator;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.TimeUnit;
@@ -273,9 +274,15 @@ abstract class AbstractConfigObject extends AbstractConfigValue implements
         } else if (stack.size() == 1) {
             return stack.get(0);
         } else {
-            AbstractConfigObject merged = stack.get(0);
-            for (int i = 1; i < stack.size(); ++i) {
-                merged = merged.withFallback(stack.get(i));
+            // to be consistent with the semantics of duplicate keys
+            // in the same file, we have to go backward like this.
+            // importantly, a primitive value always permanently
+            // hides a previous object value.
+            ListIterator<AbstractConfigObject> i = stack.listIterator(stack
+                    .size());
+            AbstractConfigObject merged = i.previous();
+            while (i.hasPrevious()) {
+                merged = i.previous().withFallback(merged);
             }
             return merged;
         }

src/test/scala/com/typesafe/config/impl/ConfigTest.scala

@@ -188,9 +188,9 @@ class ConfigTest extends TestUtils {
 
     @Test
     def mergeObjectThenPrimitiveThenObject() {
-        // the semantic here is that the primitive gets ignored, because
-        // it can't be merged with the object. But potentially it should
-        // throw an exception even, or warn.
+        // the semantic here is that the primitive blocks the
+        // object that occurs at lower priority. This is consistent
+        // with duplicate keys in the same file.
         val obj1 = parseObject("""{ "a" : { "b" : 42 } }""")
         val obj2 = parseObject("""{ "a" : 2 }""")
         val obj3 = parseObject("""{ "a" : { "b" : 43, "c" : 44 } }""")
@@ -198,8 +198,13 @@ class ConfigTest extends TestUtils {
         val merged = merge(obj1, obj2, obj3)
         assertEquals(42, merged.getInt("a.b"))
         assertEquals(1, merged.size)
-        assertEquals(44, merged.getInt("a.c"))
-        assertEquals(2, merged.getObject("a").size())
+        assertEquals(1, merged.getObject("a").size())
+
+        val merged2 = merge(obj3, obj2, obj1)
+        assertEquals(43, merged2.getInt("a.b"))
+        assertEquals(44, merged2.getInt("a.c"))
+        assertEquals(1, merged2.size)
+        assertEquals(2, merged2.getObject("a").size())
     }
 
     @Test