[DomCrawler] Added auto-discovery of namespaces in Crawler::filter() and Crawler::filterByXPath().

Improved content type guessing.
[DomCrawler] Made sure only the default namespace is removed when loading an XML content.
[DomCrawler] Added more tests for namespaced filtering.
[DomCrawler] Made that default namespace is no longer removed when loading documents with addXmlContent().
[DomCrawler] Added support for an automatic default namespace registration.