Verifying null objects only respond to defined methods.

* Delegated logic from the double to the proxy, since it is near by and
  already knows how to check if a method is implemented.
* No additional documentation, since this is the least surprising
  behaviour.
* Not sufficient duplication to warrant DRYing up the verifying method_missing
  implementations, if these classes start to look any more alike I would
  consider it though.
* verify_method_missing is a terrible method name, but I wanted to tell
  rather than ask rather than including the null object conditional in
  the method_missing implementation.

Fixes #392.