Drop an invalid Last-Modified header value returned by a FCGI/CGI

script instead tranforming it to Unix Epoch.

This bug was mentioned in the users@ mailing list and outlined in
the following centos bug: https://bugs.centos.org/view.php?id=10940
To reproduce the issue it is sufficient to connect mod-fastcgi
to a PHP script that returns a HTTP response with
the header "Last-Modified: foo". The header will be modified by
script_util.c to "Last-Modified: Thu, 01 Jan 1970 00:00:00 GMT".
Dropping an invalid header in this case seems to be the most
consistent and correct option in my opinion, plus it shouldn't
break existing configurations. Returning Unix Epoch might be
dangerous and should be avoided, but please let me know your opinions.
Moreover this is my first commit outside the documentation court,
I hope to have got the procedure right.
This fix has been tested also with the 2.4.x branch.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1748379 13f79535-47bb-0310-9956-ffa450edef68