fastcgi headers need unmangling

Headers as mangled in the FastCGI protocol, but the `getHeader()` method on `Transport` assumed the header was the original value.

Closes #1359

Reviewed By: @alexmalyshev

Differential Revision: D1098954
Make FastCGI less magical for headers

When I changed `getHeaders()` to be the same for fastcgi as all the other transports I didn't notice julk has put in special rules to deal with the fact they weren't the same. Rip that out.

This makes `$_SERVER` contain `HTTP_HOST` instead of just `HOST`.

Reviewed By: @sgolemon

Differential Revision: D1100779
bump version to 2.3.1