Fetch attribute value

[JDrizzy]

After setting up an AD-FS SSO and trying to pass a users first and last name through attributes, I found that AD-FS passes the following attribute names which are annoying/ugly to match:

# Attributes
{
  'http://schemas.xmlsoap.org/ws/2005/05/identity/claims/givenname' => ['Jarrad'],
  'http://schemas.xmlsoap.org/ws/2005/05/identity/claims/surname' => ['M'],
}
saml_response.attributes['http://schemas.xmlsoap.org/ws/2005/05/identity/claims/givenname']
# Jarrad

I thought a #fetch might be ideal here to handle cases where one might want to retrieve an attribute value using regular expression?

# Attributes
{
  'http://schemas.xmlsoap.org/ws/2005/05/identity/claims/givenname' => ['Jarrad'],
  'http://schemas.xmlsoap.org/ws/2005/05/identity/claims/surname' => ['M'],
}
saml_response.attributes.fetch(/givenname/)
# Jarrad
saml_response.attributes.fetch(/surname/)
# M

Happy to hear your thoughts ✌️

[coveralls]

Coverage increased (+0.008%) to 97.892% when pulling 1ae0c82 on JDrizzy:master into 811618d on onelogin:master.

[pitbulk]

I believe that makes sense. Can you update the documentation of the Readme as well?