Ask CGI to not warn about param() in list context.

Mason protects from the class of param() bugs which allow users to sneak in
arguments as described here:
http://blog.gerv.net/2014/10/new-class-of-vulnerability-in-perl-web-applications/

Since CGI.pm 4.05 the only way to quiet this warning is by setting their
variable as documented here
https://metacpan.org/pod/CGI#Fetching-the-value-or-values-of-a-single-named-parameter

Mason has always allowed foo=1&foo=2 to end up available in a template
as @foo = (1,2) so retain backcompat.

Quiets a warning in t/14-cgi.t tied to faking up CGI.pm