Spelling and formatting fixes for Httplib Module docs.
Improve the layout of the Httplib Module response method documentation.
It's req.ToXml, not req.ToXML.

See https://github.com/astaxie/beego/blob/develop/httplib/httplib.go
for reference.
Merge pull request #156 from cr7pt0gr4ph7/httplib-pull-request

Improve Httplib Module documentation.