[28785] introduced a bug, where encoded query arguments were decoded by `parse_str()` inside of `wp_parse_str()` but never re-encoded later on.

This encodes them, adds unit test.

Props obenland.
Fixes #29636.


git-svn-id: https://develop.svn.wordpress.org/trunk@30133 602fd350-edb4-49c9-b593-d223f7449a82