Bugfix: current code wasn't able to parse JSON repsonses other than UTF-8, UTF-16, UTF-32

According to the Apple spec (https://developer.apple.com/library/mac/#documentation/Foundation/Reference/NSJSONSerialization_Class/Reference/Reference.html), data given to [NSJSONSerialization JSONObjectWithData:..] MUST be encoded with one of UTF-8, UTF-16LE, UTF-16BE, UTF-32LE, UTF-32BE.
Anything else won't work.
The current code is using the response encoding for building the NSData from the responseString, so parsing JSON from an URI which uses an encoding other than the 5 listed above will fail (e.g. ISO-8859-1).