Add support for API change to update base branch of a PullRequest

[ryangribble]

As per this recent anouncement the API now supports changing the base branch of an existing pull request (woohoo!!!).

The Update a pull request endpoint now accepts a base string field on the request to do this.

For octokit.net this is pretty simple - we just need to add the Base string property to our existing PullRequestUpdate request class and update a few tests