Propagate key of a @cached_method correctly
Trac #16337: key parameter of cached_method is not correctly propagated

In the following example, the last line is incorrect:
{{{
sage: class Foo:
....:     @cached_method(key=lambda self,y: y+1)
....:     def f(self, y):
....:         return y - 1
sage: class Bar:
....:     f = Foo.f

sage: b = Bar()
sage: b.f(0)
-1
sage: b.f.cache
{((0,), ()): -1}
}}}

It appears that I forgot to propagate the `key` parameter in some places
while working on #15657.

URL: http://trac.sagemath.org/16337
Reported by: saraedum
Ticket author(s): Julian Rueth
Reviewer(s): Simon King