Allow to set environment variables using mod_rewrite without

Allow to set environment variables using mod_rewrite without
explicitely giving a value.

Most modules only check presence of a variable, not the
value, so it makes sense to make the VAL argument in
the mod_rewrite ENV flag optional.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@960233 13f79535-47bb-0310-9956-ffa450edef68