Use only the post parameters when calculating the signature for a request from Mandrill. Fix inspiration comes from issue on original repo (https://github.com/evendis/mandrill-rails/issues/11).
Stub out correct method in specs (request.request_parameters) that is now being to get the params for doing signature check.
Merge pull request #16 from shinydevelopment/master

Fix for authentication errors when additional params are in the parameters hash.