Removed unnecessary willChangeValueForKey and didChangeValueForKey method calls.
Fix a bug where [AFURLConnectionOperation pause] did not correctly reset
the state of AFURLConnectionOperation, causing the Network Thread to
enter an infinite loop.

The logic for 'pause' was calling [self.connection performSelector:...].
If 'pause' is executed immediately after 'start', and 'start' did not
have a chance to create self.connection on the Network Thread, then the
logic in 'pause' would effectively be a no-op. When 'resume' gets
called, the previous connection was not actually cancelled, so there are
2 connections both of which have a reference to the same NSOutputStream.
The first connection to finish/error would close the NSOutputStream, and
the second connection would spin forever in the delegate callback
connection:didReceiveData: since [self.outputStream hasSpaceAvailable]
always returns NO.

The test case I added reproduces the failure, but is flaky because it
relies on a race condition between the main thread and the Network
Thread.
Fix a bug where [AFURLConnectionOperation cancel] does not set the
appropriate error on the NSOperation.

There is a race condition where 'cancel' is called immediately after
'start'. When operationDidStart is then called on the Network Thread,
self.connection is not instantiated, because the NSOperation is
cancelled, and 'finish' is called, but without the error set.  When
cancelConnection is then called on the Network Thread, the error is not
set, because the operation is already finished.

In the new code, cancelConnection is always responsible for setting the error,
and then calling 'finish'. operationDidStart never attempts to call
'finish'.

This bug was one of the sources of non-deterministic failures in
testThatCancellationOfRequestOperationInvokesFailureCompletionBlock.
Fixed refs to AFJSONSerializer -> AFJSONResponseSerializer
Removing .swp files from example
Remove deletion of all task delegates in URLSessionDidFinishEventsForBackgroundURLSession:

This should not happen because sometimes this method is called after waking the app up to
respond to authentication challenges and no tasks have completed yet.  It is not safe to
assume that all tasks are complete and the delegates should be removed.  Doing some
results in orphaned tasks with AF delegates to handle them.

Task delegates already get deleted after the URLSession:task:didCompleteWithError: call
which will be made for each task.

Issue: 1579
[Issue #1585] Documenting default Accept header field value for setImageWithURL:, et al.
Fixed issue where the query string serialization block was not used
Merge pull request #1548 from maxgoedjen/master

Fixed refs to AFJSONSerializer -> AFJSONResponseSerializer
Merge pull request #1552 from troupe/master

Removing .swp files from example
[Issue #1552] Removing remaining references to .swp files in project
Adding spaces to image/* to avoid warning about /* inside comment block
[Issue #1483] Break out of loop if output stream does not have space to write bytes
Merge pull request #1542 from MindaugasV/master

Removed unnecessary KVO change calls
Merge pull request #1593 from kcharwood/query_string_fix

Fixed issue where the query string serialization block was not used
Merge pull request #1582 from jeremymailen/issue/1579

Remove deletion of all task delegates in URLSessionDidFinishEventsForBac...
Merge pull request #1544 from pavolas/master

Infinite loop on network thread
Merge pull request #1545 from pavolas/cancel_bug

Fix a non-deterministic failure in [AFURLConnectionOperation cancel]
Minor formatting
[Issue #1592] Adding NSParameterAssert for internal task constructors in order to catch invalid constructions early