MatchesDict crashes ungracefully non-dictionary values

[PiDelport]

Currently, the MatchesDict matcher simply crashes when matching against a value that is not a dictionary:

>>> assert_that(None, MatchesDict({1: Equals(2)}))
TypeError: 'NoneType' object is not iterable

>>> assert_that([], MatchesDict({}))
AttributeError: 'list' object has no attribute 'keys'

This makes it unnecessarily hard to diagnose tests that fail because an expected dictionary was not actually a dictionary, especially when using nested compound matchers against structures.

Would it make sense to add a check to make MatchesDict raise a MismatchError for non-dictionary-like values, instead?