Fixed in PR #1067

Following on from issue #1053, I looked into the possibility of further refactoring the codebase to remove redundant implementations.

I discovered that most of the code within ActionAssertions, FunctionAssertions and AsyncFunctionAssertions is common and can be shared with a parent class which deals with delegates.

I also discovered plenty of redundant initialisations of "Subject" for subclasses of ReferenceTypeAssertions which can be easily replaced with a single initialisation and a base class parameter.

There were also a lot of cases of error messages for similar things being implemented with slightly different wording in different areas. Fixing this issue will resolve that problem too.