Fix file check if the url contains "?"

The file check does not handle query parameters well - if the url contains one the file is not found and the asset is not rewritten. This fix tries to strip the "?" and rechecks the file.

By the way, I think that the configuration parameter "rewrite_if_file_exists" is miscalled. It should rather be a "rewrite_only_if_file_exists". Saves some looking in the doc ;)