Improve the FCGI/CGI Last-Modified header value handling.

Patch from Yann after a discussion on the dev@ mailing list. 
ap_scan_script_header_err_core_ex is now using apr_date_parse_rfc 
in order to recognize non-GMT datestr following RFC822/1123
and transforming them to GMT rather than replacing the value
with GMT now (that could add httpd's processing time to the
original value). Logging has also been improved from my initial
solution.
 


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1751138 13f79535-47bb-0310-9956-ffa450edef68