FIX bug with add_provider_to_user and authentication class in CamelCase

FIX #382

When authencatcation class is CamelCased add_provider_to_user tries to
call class.name.downcase.pluralize which leads to an error (camelcases
association instead of camel_cases)
Added CHANGELOG entry for #382 fix
Merge pull request #486 from makaroni4/master

FIX bug with add_provider_to_user and authentication class in CamelCase