[URL Operation] Use the ivar directly in initialization

This is to prevent calling the custom setter inside initialization which
broadcasts KVO notifications and relies on existing state which isn't
yet setup.

Re #1845
[URL Operation] Don't allow transitioning to invalid states
Added test to check content type
Explicitly importing MobileCoreServices.
Fixes failing test
Use SecTrust return status code at runtime

The `SecTrust` functions in use within AFSecurityPolicy.h only make use of the return code when `NS_BLOCK_ASSERTIONS` is not defined. The use of NS_BLOCK_ASSERTIONS to silence static analyzer warnings about unused values is convenient, however the larger issue is the failure of `AFPublicKeyForCertificate`, `AFPublicKeyTrustChainForServerTrust` and `AFServerTrustIsValid` to detect a runtime status code that is not `errSecSuccess` and to return early.
[Issue #1799] Implement respondsToSelector for NSURLSession delegate methods to conditionally respond to optional selectors if and only if a custom block has been set
Adding missing call to delegate in URLSession:downloadTask:didResumeAtOffset:expectedTotalBytes:
Merge pull request #1878 from kylef/state_setter

[URL Operation] Use the ivar directly in initialization
Adding necessary diagnostic ignore #pragma statements
Merge pull request #1896 from dtorres/MIMETYPE-TEST

Added test to check content type
[Issue #1961] Changing HTTP manager method signatures to accept parameters as id type
Merge branch 'master' of github.com:AFNetworking/AFNetworking
Merge pull request #1962 from RobotsAndPencils/SecTrustErrorHandling

Use SecTrust return status code at runtime
[Issue #1962] Refactoring security policy to use assertion macros to DRY up cleanup code