mod_session: Always decode session attributes early.

Backport r1850947 from trunk
Submitted by: hwibell
Reviewed by: hwibell, covener, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1851409 13f79535-47bb-0310-9956-ffa450edef68