Replace KVO (which was crashing) with swizzling.
Merge remote-tracking branch 'real/master'

* real/master:
  Removing charset parameter from Content-Type HTTP header field value
  Adding documentation for AFJSONRequestSerializer and AFPropertyListSerializer
  [Issue #2403] Fixing documentation for response serializers
  Copy editing header documentation
  Changing validatesDomainName to readonly property
  Rewriting background session manager warning, duplicating across URL and HTTP session manager
  Document expected session manager usage for background sessions
  Update .cocoadocs.yml
  [Issue #2346] Revert commits related to __has_feature(attribute_availability_app_extension)
  [Issue #2346] Conditionally compile AFNetworkActivityManagerTests
  [Issue #2346] Fix preprocessor directive to pass if __has_feature is unavailable
  [Issue #2346] Adding preprocessor directives around UIApplication network activity indicator
  [Issue #2346] Refactoring preprocessor directives to remove reference to AF_APP_EXTENSION
  Define AF_APP_EXTENSION based on availability with the preprocessor
  Update UIButton+AFNetworking.m
  [Podfile] Update to explicitly define sources
  Adding test for HTTPS request
  Squashed commit of the following:
  Added proper checks for observing AFHTTPRequestSerializerObservedKeyPaths
  Update README.md
Merge pull request #2411 from tangphillip/master

Replace AFURLSessionManager's crashing KVO with method swizzling