Wrong handling of query string when stripping parameter q

While working with ubercart for the first time I ran into a situation when securepages_get_query() produces errors. The reason is that query string contains destination[]=SOME_DESTINATION and parse_str() turns destination[] into an array instead of a string containing "[]".

I attached a patch that uses another strategy to strip "q" from query string. Additionally my patch replaces urlencode() by rawurlencode() because that's what used by drupal_urlencode().