Set the default attribute NameFormat to NAME_FORMAT_UNSPECIFIED

[ghost]

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes? (tests written by @c00kiemon5ter)
• Does your submission pass tests?
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Explanation:

Section 2.7.3.1 of the spec states that:

If no NameFormat value is provided,the identifier urn:oasis:names:tc:SAML:2.0:attrname-format:unspecified (see Section8.2.1) is in effect.

However currently pysaml2 is interpreting such attributes as urn:oasis:names:tc:SAML:2.0:attrname-format:uri. This simply sets the default to NAME_FORMAT_UNSPECIFIED and in doing so addresses #601

[ghost]

Updated to use and pass tests written by @c00kiemon5ter. Had to update the validUntil year in the test XML files as anything later than 3001 wasn't recognized as a valid parameter on Windows.

Sorry for the delay, it took me a while to be able to get my Windows environment set up to be able to run the tests without tempfile errors.

[c00kiemon5ter]

Thanks @Canderson90