Address review feedback.

* Remove redundant `@object` assignment in `VerifyingPartialMockProxy`.
* Re-use `@original_method` in `VerifyingExistingMethodDouble`. This not
  only works correctly for normal objects and `method_missing`, but
  caches the method for use later in `define_proxy_method`. If no method
  is found this code path will raise, so `find_original_method` should
  only be called once per object.
* Did not reuse `LoadedClass` in specs, since there is no need to use a
  global to test this behaviour. Since verification is being added to
  more and more places, I'd prefer to keep the specs local to the
  feature being extended rather than lumping them all in the same spec.
Extract prevents spec method to a common helper.
s/verify_partial_mocks/verify_partial_doubles/
Add feature for verify_partial_doubles.