Improves correctness of dictionary equivalency assertions, incld. respecting declared type

This brings the same behavior collections have relating to respecting declared type to dictionaries.  I mentioned in #141 that these should be handled as well, though it has taken me some time to get the time to complete the implementation.

This also corrects an edge case where some dictionary implementations were being treated as collections (rather than dictionaries) because IDictionary<TKey, TValue> does not implement IDictionary.
It is probably worth considering whether I made the right decisions with respect allowable type variance of the keys.