remove signature element for a log out request

SAML 2.0 spec requires the signature element be removed from
the request body.  This commit removes the signature in the body.

See lines 578-579 in http://docs.oasis-open.org/security/saml/v2.0/saml-bindings-2.0-os.pdf
Merge pull request #20 from ncloward/original_version

remove signature element for a log out request