Expand the usability of `Contains.Key(...).WithValue(...)`

[RenderMichael]

I want to assert on HTTP response headers, but the DictionaryContainsKeyValuePairConstraint created by Contains.Key(...).WithValue(...) does not work on the type.

Currently the constraint checks for IDictionary and (despite the error message) also IDictionary<TKey, TValue>.

However, the HttpHeaders type implements IEnumerable<KeyValuePair<string, IEnumerable<string>>> (and of course the non-generic IEnumerable interface implicitly) and nothing else.

Would it be acceptable to expand the checks on this contraint to allow IEnumerable<KeyValuePair<TKey, TValue>> as well? I'm happy to work on it if the change would be accepted.

[RenderMichael]

It's worth noting that Contains.Item(new KeyValuePair<string, Enumerable<string>>(key, value)) does the job. However, on top of being unintuitive, hard to type and read, and un-NUnit-like, it doesn't say whether the key was found or not.

It would be helpful to see whether the key was found with a different value, or the key was not found to begin with (which is outside the scope of Contains.Item).

[manfred-brands]

@RenderMichael I have no objection to add support for IEnumerable<KeyValuePair<TKey, TValue>> to Contains.
Go ahead

[RenderMichael]

Thanks. Opened #4854