Applying SOAP binding with signing enabled does not work as expected

[mheuwes]

IDP.apply_binding for SOAP with sign=True requires a prepared message (a signature element already inserted) although it requires a sig_alg parameter. Letting apply_binding sign the request provides for unique namespace identifiers.

Code Version

master

Expected Behavior

I would expect that IDP.apply_binding is correctly signing the passed message using the sig algo provided.

Current Behavior

Either a crash like #595 when passing a string as message, or crashing with saml2.sigver.SignatureError
error=Error: failed to find default node with name="Signature"

Possible Solution

Either document this behaviour or add a signature element to soap_message in httpbase.py line 317.

Steps to Reproduce

1. Create logout request:

reqid, req = IDP.create_logout_request(
    destination=destination,
    issuer_entity_id=None,
    name_id=name_id,
    session_indexes=[sess_idx],
    sign=False,
)

2. Apply binding

info = IDP.apply_binding(binding, req, destination=destination, relay_state=None, response=False, sign=True)

[c00kiemon5ter]

closing this as it should be fixed as part of #834