Fix AttributeError and signature mangling during construction of SOAP request

[mheuwes]

Currently, sending SOAP requests using pysaml2 is near-impossible, because two bugs exist:

1. IDP.apply_binding for soap without sign=True and a pre-signed Message mangles/destroys the signature by removing the line-ends within the signature.
2. IDP.apply_binding's implementation of signing using use_soap does not work currently, because instead of node_name, class_name is passed as a parameter to securitycontext.sign_statement, and is raising an AttributeError
   These two bugs are fixed with this PR

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