Test and fix for call_through w/ instance methods

[lreeves]

This adds a test case to spy on a method for any instance of an object while still returning the results of the original implementation:

Spy.on_instance_method(Foo, :bar).and_call_through

The bug is from the Proc that handles calling through not having a reference to the object receiving the method call; to fix that I've split the implementation Proc into two and adds the object instance in the call as appropriate.

[coveralls]

Coverage decreased (-0.1%) to 30.947% when pulling 651ec6b on lreeves:master into 753e3ab on ryanong:master.

[ryanong]

Thanks for the PR. This code is getting a bit unweidly I am going to try and take a stab at simplifying it a bit

[lreeves]

Sounds good, let me know if you need anything.