leading "?" removed from query strings passed through add_query_arg()

[markjaquith]

example:

$test = '?s=test&paged=2';
$test = remove_query_arg('paged', $test);
echo $test;

expected result:

?s=test

actual result:

s=test

(In [5703]) only strip question marks from the RIGHT side of the query string. fixes #4464. see #4462

(In [5704]) only strip question marks from the RIGHT side of the query string. fixes #4464 for trunk