Support content negotiation in WebAPIResource.

This introduces support for HTTP Accept-based content negotiation in
WebAPIResource. By default, any request to a WebAPIResource will return
either XML or JSON data, depending on the HTTP Accept header in the request.
The default is JSON. This only happens by default for resources that return
a tuple of data, rather than a specific object.

It also fixes up a few problems when providing XML results. Namely, when
serializing a dictionary where keys are non-strings, or when serializing
tuples.

Tested by using curl with different mimetypes.