Sometimes it's useful to spy on a function and override it (to influence

or track some state), but to then be able to invoke the original

behavior.
This change implements this through a new call_original() function,

which will invoke the original function's bytecode. It will pass any

positional or keyword arguments to the original function, taking care to

pass the correct self or cls for bound/class methods. The call is

not logged, so it won't impact any spy tracking.
Examples of how to use this new functionality can be found in the

README.

Unit tests pass for Python 2.7, 3.5, 3.6, and 3.7.