search() should take a QName for "language", not a string

[Googulator]

In WebDAV, the search "language" (identified by the XML tag name in the search request) is generally not in the "DAV:" namespace, but can be in any other namespace. To support this, search() should take a QName for the "language" parameter, not just a simple string, and "DAV:" shouldn't be hardcoded as the namespace for the "language" tag.