Removing old rails 3.x glitch workaroung

This issue has beed fixed in stable rails version. I do think that maintaining this workaround is no longer necessary.

The current implementation causes trouble when server returns empty body, 204 NO_CONTENT (iOS 6 and 7):

1. data is not nil, but zero length (_NSZeroData)
- responseString is zero length (@"")
- condition is triggered
- inner data is NSConcreteMutableData
- deserialization fails

My fix solves that issue.