Added a .to_s when a hash is thrown

lib/fizzy/api/sessions/basic_auth_session.rb

@@ -47,7 +47,7 @@ module Fizzy
           when 401
             access_denied(response)
           when 500
-            raise response.parsed_response || "Received HTTP response code #{response.code}!"
+            raise (response.parsed_response || "Received HTTP response code #{response.code}!").to_s
           else
             response
           end