Update InternalBackend to send non-GET data in body.

The default backend currently passes all data in the URL query parameters.
This isn't our recommended way of making POST requests and could interact
badly with systems that e.g. assume URLs are always safe to log or assume
URLs represent resources that can be safely retrieved. This PR changes
InternalBackend to use a form-encoded HTTP body for non-GET requests.
Merge pull request #37 from metcalf/am-post-body

Update InternalBackend to send non-GET data in body.