Revert #5 (JSON 'null' should not be deserialized as a pointer)

[ChimeraCoder]

As of go 1.1, json.Unmarshal no longer produces an error when attempting to unmarshal a possibly null value into JSON.

This was patched with https://codereview.appspot.com/6759043

This was probably introduced with 01f8edd544aa78baaae7b65686963f2bfa229400, though the commit cannot be reverted cleanly.