Explicitly set URL parameter separator for external URLs

Summary:
PHP has this crazy 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