Fix depending on the method of a sub parameter object

[maximlt]

Fixes #764

This PR fixes a regression that was introduced in #753. It appeared that depending on a method was pretty much untested so I have added some more tests.

When _spec_to_obj resolves a dependency on a Parameterized method, the type of the resolved method is:

• FunctionType if it's a direct dependency, as in @param.depends('method'). The object holding the reference to the method seen by _spec_to_obj is a class.
• MethodType if it's a dependency on a sub object, as in @param.depends('sub.method'). The object holding the reference to the method seen by _spec_to_obj is an instance.

Good to know!

from types import FunctionType, MethodType

class A:
    def foo(self): pass

assert isinstance(A.foo, FunctionType)
assert isinstance(A().foo, MethodType)

The change made in this PR is pretty simple, I'll however kindly ask @philippjfr to review it to ensure that this is correct, and check that the problem actually doesn't lie with how the object holding the method reference is resolved (class vs. instance).

[philippjfr]

Thanks for the fix! The change makes sense to me, I think this wasn't originally implemented because the actual decorator is given the function before it is turned into a method.