Cover ReleaseDate underlying changes with tests

[nulltoken]

See #84

[JakeGinnivan]

Release date is moved into the SemVer class, so it gets approved with everything else, but it gets scrubbed out

But it should be easier to add these tests now

[nulltoken]

• Tests have been added.
• This PR also adds a helper to reduce the boilerplate code required when dealing with equality implementation. If you don't feel that makes sense to keep it, I'd be happy to drop it.
• Along the way, SemanticVersionPreReleaseTag was also fixed. It was lacking a GetHashCode() method.