tundra.tn.content:route - change the default content type for XML
to be 'text/xml'

The WmEDIINT package hijacks the 'application/xml' type by registering
its own content handler for it, and that content handler does unexpected
things with plain old XML content.

Also, rightly or wrongly, 'text/xml' is the idiomatic type used by Trading
Networks.
tundra.tn.content:route - add unit tests