With encryption The ID of each Signature element must be unique

If assertion is gonna be encrypted and the response and the assertion
are signed, both signatures will have an ID of `signature1` which
leads to an invalid xml because the ids have to be unique.
A similar issue has been reported and fixed the same way in server.py
with this commit: 4375361

Author: moonfriend

src/saml2/entity.py

@@ -874,7 +874,7 @@ def _response(
                         _assertion.signature = pre_signature_part(
                             _assertion.id,
                             self.sec.my_cert,
-                            1,
+                            2,
                             sign_alg=sign_alg,
                             digest_alg=digest_alg,
                         )