This follows the recommendation from issue #4181. A handler
for 'view-source' delegates to the HTTP loader. In that
loader I check for view-source, adjust the URL to be the
URL to be viewed and modify the Content-Type header to be
text/plain.

This doesn't actually result in the source being
viewed as rendering text/plain is not yet implemented.

This is done by detecting the content type as text/plain
and following the requirements from:
https://html.spec.whatwg.org/multipage/browsers.html#read-text