The RKObjectMapping -classForKeyPath: gets called a lot; avoiding the cr...

The RKObjectMapping -classForKeyPath: gets called a lot; avoiding the creation of a temporary NSArray for simple keys (the usual case) makes a measurable speed difference.  This does show up in profiling for RestKit/RestKit/issues/2065

Using NSLiteralSearch is faster than option 0 (since the default is to normalize the strings first), so I just added that.  Using rangeOfCharacterFromSet: is faster still, but that would require creating a global variable with the NSCharacterSet instance (even calling a method to get it would negate any advantage), so I didn't bother.  It might be something to consider if it can be used in a number of places, but the speedups are still pretty minor.  The big thing is to avoid the object creation.

Speed before:
(Device) Mapping 5000 students with relationship mapping: 39.823580
(Simulator) Mapping 5000 students with relationship mapping: 5.768704

After:
(Device) Mapping 5000 students with relationship mapping: 38.642747
(Simulator) Mapping 5000 students with relationship mapping: 5.517917