Make path_redirect_load() and path_redirect_save() compatible with each other

Currently there is an API difference between the $redirect variable returned from path_redirect_load() and the one used by path_redirect_save(). This causes redirects to not save properly because of a MySQL error when trying to do something like the following:

$redirect = path_redirect_load(1);
path_redirect_save($redirect);

You receive the error:

Warning: mysql_real_escape_string() expects parameter 1 to be string, array given in db_escape_string() (line 321  of /Users/nate/Sites/drupal6/includes/database.mysql.inc).

And the update fails when trying to put an array into a string placeholder.

This patch makes these two functions work better with each other, just by converting query arrays back into strings before doing the update.