Support for multiple/mixed attribute types.

[doconnor-clintel]

Revised version of https://github.com/apokalipto/devise_saml_authenticatable/pull/159

• Fixes specs
• Adds coverage for multiple groups, based off of okta samples

[doconnor-clintel]

Okay, that's positive.

This feels quite... ugly... to mutate global state in a class when a configuration flag is set.

[doconnor-clintel]

Annoying.

NoMethodError:
        undefined method `single' for {"first_name"=>["John"], "email"=>["[email protected]"], "groups"=>["admin", "reporting"], "multiple_but_single"=>["1", "2", "3"], "lastName"=>["Smith"]}:Hash

However, that should be a OneLogin::RubySaml::Attributes object with those methods.