Dynamic mapping equivalence should just check pointer identity

The existing implementation of `isEqualToMapping:` in `RKDynamicMapping` simply returns `NO` in all cases. However, if we have a non-nil `RKDynamicMapping` called `mapping`, this expression
```
[mapping isEqualToMapping:mapping]
```
should evaluate to `YES`. It is nonsensical and unexpected to return `NO` in this case. 

I understand that comparing two dynamic mappings on a matcher-by-matcher basis would take a non-trivial amount of care and work to implement correctly. So for now, we should leave this check to be simply pointer equality.