Allow nested call to setImageWithURLRequest

Calling [UIImageView setImageWithURL:] inside [UIImageView setImageWithURLRequest:placeholderImage:success:failure] completion block could NOT work.
This was due to internal af_imageRequestOperation set to nil AFTER a completion block is called. With this fix, it is now set to nil BEFORE.
Merge pull request #1184 from autresphere/master

Allow nested call to setImageWithURLRequest