Allow setting the merge strategy via the REST API

Right now, if you merge using the rest API, the default merge strategy will be used.  While this is a sensible default, it would be great if you could select other merge strategies allowed by the repo.

We use custom toolting to merge PRs that integrates with other internal tools.  We use a rebase workflow.  This is normally fine, but sometimes there are long-running branches with multiple merges, and the rebase becomes too expensive to perform manually.  In these cases, our developers must used the web interface the merge, giving up all the advantages of our custom tooling.