SAML2: Sign the response if no assertion is present.

It is possible to send an response without an assertion, such as in the case
of IsPassive. In those cases we don't have an assertion to sign, and we should
fall back to signing the response.


git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@693 44740490-163a-0410-bde0-09ae8108e29a