Avoid unneccesary expensive isEqualToString: calls and conversions

As the `-[AFJSONRequestOperation responseJSON]` method is potentially called very often, unnecessary conversions between NSData and NSString should be avoided.

Also `-[NSString isEqualToString:]` is comparably expensive. As it is only needed to find out if the response is a single space the byte length of the NSData has to be 1 (quite strange that this is not checked in `-isEqualToString:`'s implementation). 

Before: 

![before](https://f.cloud.github.com/assets/15453/987162/797e89a0-08e6-11e3-8cca-25aad3e2df9b.png)

After:

![after](https://f.cloud.github.com/assets/15453/987164/7e20c8ec-08e6-11e3-9440-87db748504c7.png)