Session: Fix missing quotes in assert-statements.

This ensures that the assert-statements gives move informative output
when they fail.
(This patch also changes the file to use utf-8 encoding.)

Thanks to Ryan Panning for providing this patch.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3066 44740490-163a-0410-bde0-09ae8108e29a