R1:d9ce80aa17af

Explicitly set URL parameter separator for external URLs

Summary:
PHP has this crazy [[ http://php.net/arg_separator.output | arg_separator.output ]] INI setting which allows setting different string for URL parameters separator instead of `&` (e.g. in `?a=1&b=2`).

Don't use it for external URLs.

Test Plan: Log in through OAuth.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2284