Fix odd edge case for POST Http::request()'s. I incidentally found this while working...

Fix odd edge case for POST Http::request()'s. I incidentally found this while working with Special:Import. For some reason that I can't 100% nail down--works fine on Win32, fails on CentOS--cURL seems to want CURLOPT_POSTFIELDS set to at least an empty string, if not values. Using a workaround I found, we'll set it to an empty string if we're making a POST. Since we explicitly allow setting of arbitrary curlopts later, users can still override the empty string if they in fact want to set this. This fixes my Special:Import problem (and possibly similar issues) and has no regressions afaict. Credit to Phill Sparks @ milk-hub.net/blog/2008/08/26/curl_error_26#comments-77