Fix a doc type; (GET x, y) is not (GET(x), y).
Return a "400 Bad Request" error if we're redirected to a URI with a bad scheme.

Sean Brunnock reported the issue when a link redirected his program to
<htttp://...>.  This triggered an internal confess() we had put there
to trap a strange case we couldn't reproduce.  His test case and patch
were instrumental in helping me resolve this on a deeper level.