Added Xunit CustomMatcher to test case of union

[b0wter]

I've added code to allow the user the check the case of a union (see #16). Example:

type MyUnion
    = One
    | Two
    | Three

First |> should be (ofCase<@ First @>) // this will succeed
Second |> should be (ofCase<@ First @>) // this will fail

Currently, this only works in Xunit. If you like the idea I'll try to add support for other frameworks.

[sergey-tihon]

Hi, thank you for the PR!

Can you please add tests for other frameworks?

• MbUnit out of scope
• MSTest should be relatively easy (reusing the same code from CustomMatchers)
• NUnit (if we can do it without NHamcrest dependency)

[b0wter]

Is there a good place to store common code?
For Xunit I simply wrapped a CustomMatcher around the two necessary methods. I guess it can be trivially done for Nunit and MSTest and I'd like to write the actual logic only once.

[b0wter]

I've updated my PR to include an implementation of ofCase for MSTest and NUnit. I've also added tests for them and updated the docs.
Since I didn't see a common code repository I've added the logic for ofCase twice (Xunit -> CustomMatchers and NUnit -> CustomConstraints).

[sergey-tihon]

Thank you for PR @b0wter !
Merged and released as v3.5.0