Preserve xmlns:xs namespace when signing and serializing responses

[cjduffett]

Fix for: #76

This PR overrides the OneLogin_Saml2_XML static method cleanup_namspaces, which is directly invoking etree.cleanup_namespaces. I added a whitelist of namespaces that should not be cleaned from an element before serialization into XML. This preserves the xmlns:xs namespace that's specifically needed to validate the schema of a SAML Response's attributes.

I also added some additional namespace prefix constants (now grouped with the other namespace constants) in OneLogin_Saml2_Constants.

[cjduffett]

@pitbulk For review please.

[coveralls]

Coverage decreased (-0.02%) to 96.004% when pulling 0e7b731 on cjduffett:bugfix/add-sign-xmlns-xs-patch into 6505e5c on onelogin:master.

[coveralls]

Coverage decreased (-0.02%) to 96.004% when pulling 0003e7c on cjduffett:bugfix/add-sign-xmlns-xs-patch into 6505e5c on onelogin:master.

[patricksanders]

Any word on this PR? We're having to work around it right now, would be nice to have this merged.

[fgao22]

Same here. Anything we can do to get this PR passed and merged?