Fix "post-redirect" not working when one of the parameters is named "submit".

It appears that there is some special processing of elements named "submit"
when triggering form-submit through javascript. This patch adds a workaround
for that.

Thanks to Christian Gijtenbeek for finding and fixing this bug!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3024 44740490-163a-0410-bde0-09ae8108e29a