RFC - accept xml content

[missedone]

hi @eriklupander
for now, gotling only support accept: json, it'd be good if can support xml as well.
i saw there is golang xmlpath implement the xpath spec.

thanks

[eriklupander]

Sounds good, xmlpath (or perhaps gpath if there is a golang implementation) should be supported.

[eriklupander]

I have pushed a first attempt at using xmlpath. Seems to work but code needs to be cleaned up, unit-tested etc.

[missedone]

it'd better to mention in README that go-xmlpath does not support xml namespace yet go-xmlpath/xmlpath#8