Fix failure with 201 status code on adding a collaborator

[annchous]

Close #2558 and close #2373 too.

I wrote such a local test:

[Fact]
public async Task AddCollaborator_ThrowsNoException()
{
    GitHubClient client = new GitHubClient(new ProductHeaderValue("name"))
    {
        Credentials = new Credentials("token")
    };

    var response = await client.Repository.Collaborator.Add("owner", "name", "user", new CollaboratorRequest(Permission.Admin));
}

There was no exception and response was valid. An old version of this method throws ApiException (this case I have also covered with the corresponding test).

[FrediKats]

@timrogers can you check plz?

[JonruAlveus]

release_notes: Fix failure with 201 status code on adding a collaborator

[JonruAlveus]

Hi @annchous, I've run the tests but there's a failure. Please could you check it out?

Octokit.Tests.Clients.RepoCollaboratorsClientTests+TheAddMethod.SurfacesAuthorizationExceptionWhenSpecifyingCollaboratorRequest

Cheers

[annchous]

@JonruAlveus There was a problem in mock configuration. Now we use ApiConnection instead of Connection. Now test passes.

[timrogers]

This is looking great to me! ✅ Are you happy too @JonruAlveus?

We'll need to release this in a new major version as a breaking change since it changes the return type of a method.

[nickfloyd]

release_notes: Fixed a failure with 201 status code when adding a collaborator