Fix Crash in RKPropertyInspector

Upgrading to the lastest dev commit of RestKit causes our app to crash.

The problem is line 125:

<pre>
        [self.inspectionCache setObject:inspection forKey:(id<NSCopying>)objectClass];
</pre>

ObjectClass is nil.  If you backtrack the call sequence you can see that rk_classForPropertyAtKeyPath is not coded correctly since at line 140 it intentionally send a nil class to classForPropertyNamed resulting in the crash.