Keep request body in same state as it was before read

When XmlParser was part of Rails 3, developer was able to access request#body without an issue.

At the moment in Rails 4 and using actionpack-xml_parser, request#body-s are handled different for XML and JSON  (XML needs rewind, JSON does not).

It is not hard before accessing request#body to call #rewind on it, but keeping same feel between different versions and request types, might be a good idea.