add_query_arg() removes blank query vars

[markjaquith]

skeltoac: can anybody explain why add_query_arg removes blank query vars?

This results in a destructive query var addition, in which existing (but blank) args disappear.

remove_query_arg() was removing by passing a blank string. Upcoming patch moves that to FALSE, which allows blank query vars to be added.

It also preserves ?foo&bar=test vs ?foo=&bar=test

In all, it just preserves the existing QS a whole lot better.

Patch for trunk

(In [4435]) Better preservation of query string when using add_query_arg(). Props in self- and Andy-flavored varieties. fixes #3308