Allow have_key/have_keys to receive another matcher as keys

[jaimegildesagredo]

The same way we allow to receive another matcher as a possible value for a dict, we should allow to receive a matcher as a possible key of the dict. For example:

expect({'foo': 1}).to(have_key(start_with('f'), 1))
expect({'foo': 1}).to(have_keys({start_with('f'): 1}))