Adds the overrideMimeType(mime) method.
Wires the final MIME type (override ?? response) into responseXML for
the default response type (""): when the final MIME is text/xml, the
body is lazily parsed into a Document and cached in a new
_response_xml field. Other XML MIME types (application/xml, image/svg+xml)
land in Mime.ContentType.other whose backing slices are unsafe to read
after Mime.parse returns; supporting them is left for a follow-up.
The override does not affect response / responseText today —
responseText charset decoding and the document responseType's
HTML-vs-XML parser choice are noted as follow-ups in the code.
bf5d43c541