If a decorator fails to copy over the function name from the wrapped

function, spying on the decorator can fail, due to the inability to

locate the correct function for the slippery function check.
This situation is now detected and a warning logged. It advises using

a new parameter, func_name=, which enables a spy to specify an

explicit function name. This is rarely needed, but useful when

encountering this sort of situation.

All unit tests pass on all supported versions of Python.