drop xhtml content-type support (closes #2065651)
* HTMLStream does not care about xml any more
* reqquest.demote_to_html and .xhtml_browser are deprecated
* web config: drop force-html-content-type option
* adjust tests
[htmlparser] store unaltered source in pageinfo.source
Do not store the parser to preserve the source aspect. Store the initial input
in the source argument instead.
This is very useful when parsing html. In such case we need to drop ``<script>``
tag.