Produce correct query strings from PhutilUri regardless of ini settings

Summary: http_build_query will use the separator set in arg_separator.output. When this is set to & (which happens in various environments) this causes query parameters to have the & doubly encoded.

Test Plan:

-> php -r 'ini_set("arg_separator.output", "&"); echo http_build_query([1, 2, 3], "", "&");'
0=1&1=2&2=3

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14279