HTTP proxy: insert slash in URL if missing

curl has been accepting URLs using slightly wrong syntax for a long
time, such as when completely missing as slash "http://example.org" or
missing a slash when a query part is given
"http://example.org?q=foobar".

curl would translate these into a legitimate HTTP request to servers,
although as was shown in bug #1206 it was not adjusted properly in the
cases where a HTTP proxy was used.

Test 1213 and 1214 were added to the test suite to verify this fix.

The test HTTP server was adjusted to allow us to specify test number in
the host name only without using any slashes in a given URL.

Bug: http://curl.haxx.se/bug/view.cgi?id=1206
Reported by: ScottJi