Properly decode JSONObject.NULL => null [Fixes #209]

ParseEncoder encodes (null) => JSONObject.NULL. But ParseDecoder was not
doing the inverse conversion.

That resulted in JSONObject.NULL values being in the ParseObject.State map,
which means any calls to State.get(key) that were casting the result to a
specific type (i.e., String) without first checking the Object return type
would result in a ClassCastException.

ParseObject already accounts for this possibility by checking the Object type
before casting it, and returning null if the type did not match.

The known case that causes crashes is #209, where ParseUser.State.sessionToken()
casts the result of get(KEY_SESSION_TOKEN) to String. When that was null it
would be encoded as JSONObject.NULL due to the ParseEncoder; but after decoding
it would not convert back to a native null, leading to the ClassCastException.

Author: jhansche

Parse/src/main/java/com/parse/ParseDecoder.java

@@ -71,6 +71,10 @@ public Object decode(Object object) {
       return convertJSONArrayToList((JSONArray) object);
     }
 
+    if (object == JSONObject.NULL) {
+      return null;
+    }
+
     if (!(object instanceof JSONObject)) {
       return object;
     }

Parse/src/test/java/com/parse/ParseDecoderTest.java

@@ -73,6 +73,12 @@ public void testNonJSONObject() {
     assertSame(obj, ParseDecoder.get().decode(obj));
   }
 
+  @Test
+  public void testNull() {
+    Object object = ParseDecoder.get().decode(JSONObject.NULL);
+    assertNull(object);
+  }
+
   @Test
   public void testParseFieldOperations() throws JSONException {
     JSONObject json = new JSONObject();