* [x] Have you checked to ensure there aren't other open [Pull Reques…

…ts](../../pulls) for the same update/change?

* [x] Have you added an explanation of what problem you are trying to solve with this PR?

It is needed for returning attributes like
````
          <saml:Attribute Name="dateOfBirth">
            <saml:AttributeValue xsi:type="xs:date">2015-12-06</saml:AttributeValue>
          </saml:Attribute>
 ````

Otherwise I get an ValueError Exception like
````
Type and value do not match: date:<class 'str'>:2015-12-06
````
because `type(value) is not valid_type == True`

I didn't write any unit test, just tested in my context.

Two important things:
1. This PR is on top of #597 but it doesn't have any dependencies on it. The only file I changed is saml.py, let me know if this PR will be merged in next releases and if you need separate branch for every features. In the future I will this way.
2. I think that the following code https://github.com/IdentityPython/pysaml2/pull/602/files#diff-6c156669cad61eda35e679329251dce9R197 could be also improved according to #518

src/saml2/saml.py

@@ -4,6 +4,7 @@
 # Generated Mon May  2 14:23:33 2011 by parse_xsd.py version 0.4.
 #
 import base64
+import datetime
 
 from saml2.validate import valid_ipv4, MustValueError
 from saml2.validate import valid_ipv6
@@ -191,6 +192,11 @@ def _wrong_type_value(xsd, value):
                 'to_type': _str,
                 'to_text': _str,
             },
+            'date': {
+                'type': datetime.date,
+                'to_type': lambda x: datetime.datetime.strptime(x, '%Y-%m-%d').date(),
+                'to_text': _str,
+            },
             'integer': {
                 'type': int,
                 'to_type': int,