Send mimetype 'text/xml' to `html_to_text`
Send mimetype 'text/xml' to `html_to_text`

According to the official standards
(https://tools.ietf.org/html/rfc7303), "The registration information for
text/xml is in all respects the same as that given for
application/xml...except that the 'Type name' is 'text'."

opensyllabus/osp-pipeline,opensyllabus/osp-pipeline