support "on" and "off" when converting string to boolean

HOCON.md

@@ -652,11 +652,11 @@ implementation should attempt to convert types as follows:
    representation that would be a valid number in JSON.
  - boolean to string: should become the string "true" or "false"
  - string to number: parse the number with the JSON rules
- - string to boolean: the strings "true", "yes", "false", "no"
-   should be converted to boolean values. It's tempting to
-   support a long list of other ways to write a boolean, but
-   for interoperability and keeping it simple, it's recommended to
-   stick to these four.
+ - string to boolean: the strings "true", "yes", "on", "false",
+   "no", "off" should be converted to boolean values. It's
+   tempting to support a long list of other ways to write a
+   boolean, but for interoperability and keeping it simple, it's
+   recommended to stick to these six.
  - string to null: the string `"null"` should be converted to a
    null value if the application specifically asks for a null
    value, though there's probably no reason an app would do this.

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

@@ -31,9 +31,10 @@ final class DefaultTransformer {
                     return new ConfigNull(value.origin());
                 break;
             case BOOLEAN:
-                if (s.equals("true") || s.equals("yes")) {
+                if (s.equals("true") || s.equals("yes") || s.equals("on")) {
                     return new ConfigBoolean(value.origin(), true);
-                } else if (s.equals("false") || s.equals("no")) {
+                } else if (s.equals("false") || s.equals("no")
+                        || s.equals("off")) {
                     return new ConfigBoolean(value.origin(), false);
                 }
                 break;

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

@@ -422,4 +422,17 @@ class ConfigValueTest extends TestUtils {
         assertEquals(Int.MaxValue + 1L, nD(Int.MaxValue + 1.0).unwrapped())
         assertEquals(Int.MinValue - 1L, nD(Int.MinValue - 1.0).unwrapped())
     }
+
+    @Test
+    def automaticBooleanConversions() {
+        val trues = parseObject("{ a=true, b=yes, c=on }")
+        assertEquals(true, trues.getBoolean("a"))
+        assertEquals(true, trues.getBoolean("b"))
+        assertEquals(true, trues.getBoolean("c"))
+
+        val falses = parseObject("{ a=false, b=no, c=off }")
+        assertEquals(false, falses.getBoolean("a"))
+        assertEquals(false, falses.getBoolean("b"))
+        assertEquals(false, falses.getBoolean("c"))
+    }
 }