adding autorelease pool for responseObject

This is a really small fix.
According to my profiling, adding `@autorelease` around parsing of the `responseObject` lowers the peek memory consumption dramatically (depending on the response size and chosen serializer).
I checked it with `AFJSONResponseSerializer` and a custom JSON-based response serializer creating Mantle-based model objects.
Let me know if this makes sense.
Thank you.