Adding runtime hack for supporting the file:/// scheme.

Back when AFNetworking decided to support the file:/// scheme, I suggested this runtime hack to future proof this scenario but got rejected because "I was overthinking it". After reading #1146 and #1155 where it seems people are using the file scheme for images which AFInflatedImageFromResponseWithDataAtScale can't handle, I feel its time to rethink using this hack. By merging this pull request, AFNetworking will never have to deal with statements like `[response respondsToSelector:@selector(allHeaderFields)]` again (which can easily be forgotten).