Added capability to assert that a generic dictionary with string key and custom equality comparer contains key.

IDictionary<string, string> dictionary = new
ConcurrentDictionary<string, string>(StringComparer.OrdinalIgnoreCase);
dictionary.Add("name", "foo");

//The following 2 lines assert the same and should both pass
dictionary.ContainsKey("NaMe").Should().BeTrue(); //passes
dictionary.Should().ContainKey("NaMe"); //failed before

The fix assures that the second line passes also.