Properly decode JSONObject.NULL => null [Fixes parse-community#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 parse-community#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();