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