fix a crash when using BlocksKit (caused by NSProxy)

If obj is a instance of subclass of `NSProxy`, when calling class_getName([obj class]) will cause `EXC_BAD_ACCESS` crash.
The case I met is when `A2DynamicDelegate` instance from [BlocksKit](https://github.com/zwaldowski/BlocksKit/blob/master/BlocksKit/DynamicDelegate/A2DynamicDelegate.h) is passed to `static inline bool canRecordObject(Class cls);` method, it crashes on the line `const char *name = class_getName(cls);`

Try to fix it by simply not testing its class, and just ignore it.