SAML2_Binding: Allow the use of a POST request for transmitting artifacts.

The specification allows the SAMLart parameter to be sent as POST data.
Update the code to allow this.

Thanks to Ofer Gigi for locating this bug.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2819 44740490-163a-0410-bde0-09ae8108e29a