Add a RestClient variant that sends and expects XML

For now, this leaves the base class as defaulting to JSON and
just adds RestClientXML as a variant that does XMLish things.

This adds a headers argument to get and delete so that we can
declare to the API that we speak XML.

Also, add some common XML utilities.

Change-Id: I883de8e21ae18eed929705cff49b6dfb112d20c2
2 files changed