Explicitly state our argument separator when calling http_build_query().

[coling]

When the PHP arg_separator.input configuration option is not set to & (e.g. if it's set to &) then this will result in malformed encoded strings. Sometimes this is generally OK, but when using http_build_query() to format POST data (e.g. in the Http/Transport classes) this can result in malformed POSTs and failing APIs

This is easily seen in e.g. reCaptcha verification requests.

Summary of Changes

This is a long overdue rebase of #23616

Testing Instructions

Simply set arg_separator.input to & in php.ini and attempt to use reCaptcha

Actual result BEFORE applying this Pull Request

It will fail with a malformed request

Expected result AFTER applying this Pull Request

It will work.