Removing automatic XML building from http requests in the Xml class

This has a number of reasons:

* There were no tests for this piece of code
* We recommend using Xml::build for request input, this could be a security risk
* Creates an annoying dependency to the Network namespace
* Pre-configuring the HTTP client is impossible
* It is much cleaner and simple to just pass the response body into the function
Merge pull request #4516 from cakephp/3.0-cleanup-xml

Removing automatic XML building from http requests in the Xml class