Enable RSpec/HookArgument cop and fix issues

To be consistent about before/after arguments, as `:each` or `:example`
 are same and default scopes, best not to send an argument in those
 scenarios.

Read about cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/HookArgument

spec/controllers/users/registrations_controller_spec.rb

@@ -4,7 +4,7 @@ describe Users::RegistrationsController do
 
   describe "POST check_username" do
 
-    before(:each) do
+    before do
       @request.env["devise.mapping"] = Devise.mappings[:user]
     end