Unnecessary XML validation

XML validation against a schema is not required within Play and could present an unnecessary performance overhead. Places where XML validation is performed are:

* L422 of ContentTypes.scala
* L662 of WS.scala

Recommendation in terms of establishing a loader:

    val f = javax.xml.parsers.SAXParserFactory.newInstance()
    f.setValidating(false)
    val p = f.newSAXParser()
    val nonValidatingXml = xml.XML.withSAXParser(p)

...and when required

    val doc = nonValidatingXml.load(url)