Upgrade Rails to version 5.2.1 and adopt code & specs where necessary

Signed-off-by: Sebastian Serth <Sebastian.Serth@student.hpi.de>

app/controllers/error_template_attributes_controller.rb

@@ -81,6 +81,6 @@ class ErrorTemplateAttributesController < ApplicationController
 
     # Never trust parameters from the scary internet, only allow the white list through.
     def error_template_attribute_params
-      params[:error_template_attribute].permit(:key, :description, :regex, :important)
+      params[:error_template_attribute].permit(:key, :description, :regex, :important) if params[:error_template_attribute].present?
     end
 end