When queries are passed to `WP_Tax_Query`, the value for `operator` is assumed to be uppercase. This is an unnecessary constraint. In `->get_sql()`, cast it using `$operator = strtoupper( $query['operator'] )`.

Props clifgriffin.
Fixes #27557.


git-svn-id: https://develop.svn.wordpress.org/trunk@28618 602fd350-edb4-49c9-b593-d223f7449a82