kohabug 2078 - send correct Content-type for search feeds

kohabug 2078 - send correct Content-type for search feeds

OPAC search RSS and ATOM feeds now have the correct
Content-type sent - "application/rss+xml" and "application/atom+xml",
respectively.

As part of this patch, added an optional fourth parameter
to C4::Output::output_html_with_http_headers to specify
the content type.  If that parameter is now supplied, or if
the value of the parameter does not contain at least a "/",
the default type of "text/html" is returned.

No documentation changes.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>