removesKeysWithNullValues in nested arrays

(previously only in dictionaries)
[Issue #2008] Fixing detected error domain for response serializers
Merge pull request #2007 from jilouc/master

JSON response serializer: removesKeysWithNullValues of objects in nested arrays
Minor formatting
[Issue #2007] Respecting NSJSONReadingMutableContainers reading option in AFJSONObjectByRemovingKeysWithNullValues
[Issue #1884] Don't override Content-Type header value if specified in request serializer HTTPRequestHeaders property
Bumping version to 2.2.3
Update README.md
Update README.md
[Issue #2032] Documenting change in behavior of NSOutputStream property
Merge branch 'master' of github.com:AFNetworking/AFNetworking
[Issue #2032] Only nil out outputStream property of connection operation if responseData is non-nil, meaning that no custom  was set