Fix #140: make 'path' required for Connection.api_request.

After consideration, I don't believe there is any sensible default value for 'path'.

See #140.
That change ended up on my 'master' branch before I created the branch for this PR.  It will land either here, or in PR #192 (fixing #138).  I doubt it is worth the trouble to figure out how to edit the changeset out of this PR's branch.