Add a namespace for AttributeValue xsi:type value

[ashimaathri]

This fixes the issue when integrating with onelogin's python3-saml client:

Element '{urn:oasis:names:tc:SAML:2.0:assertion}AttributeValue', attribute '{http://www.w3.org/2001/XMLSchema-instance}type': The QName value 'xs:string' has no corresponding namespace declaration in scope.

[ashimaathri]

Oops! This fixes the same issue as #336, but in a different way.

[jaustinpage]

Either #336 or this pr should be merged, not both. both look good to me 👍

[jaustinpage]

👍

[jaustinpage]

Either #336 or this pr should be merged, not both. both look good to me 👍

[jgehrcke]

1. I think it's cleaner to just conditionally add this attribute, as is done in AttributeValueBase: include xs namespace definition in set_type #336.

2. This PR here sets self._extatt['xmlns:xs'] in addition to self.extension_attributes["xmlns:xs"]. In my tests (when I was working on AttributeValueBase: include xs namespace definition in set_type #336) this was not required and it is too long ago for me to understand the difference. @ashimaathri are you able to explain, hopefully having this at the top of your head? :-)

[jaustinpage]

https://github.com/rohe/pysaml2/blob/master/src/saml2/__init__.py#L386 does it make sense to use the setter?

[jaustinpage]

@jgehrcke it looks like a fallback. The access that ashima is doing looks consistent with other code in here... https://github.com/rohe/pysaml2/blob/master/src/saml2/saml.py#L241

self._extatt gets set on the object init to be empty. self.extension_attributes gets set to this empty object. https://github.com/rohe/pysaml2/blob/master/src/saml2/saml.py#L140 (i am guessing this is for legacy use of this object)

self.extension_attributes is from SamlBase, the class this extends. https://github.com/rohe/pysaml2/blob/master/src/saml2/__init__.py#L302

[ashimaathri]

@jgehrcke Thanks for you comment. I've updated my code to conditionally add this attribute. And it looks like @jaustinpage has answered your other questions. I'm ok with either PR getting in as long as the issue is fixed.