OauthClient.GetGitHubLoginUrl returns incorrect URL for Enterprise instances.

[grokys]

OauthClient.GetGitHubLoginUrl with an enterprise instance URL of https://example.com should return e.g. https://example.com/login/oauth/authorize?client_id=secret, and indeed that is what the unit tests are testing.

In real usage, however, connection.BaseAddress will return a URL like https://example.com/api/v3/ meaning that the returned URL will be https://example.com/api/v3/login/oauth/authorize?client_id=secret - this URL is wrong.