Remove non-lowercase headers in Rails default configuration (fixes #541)

While this gem now uses lowercase headers, the Rails default configuration still
defines non-lowercase headers.  As a result, our Railtie will not remove those
conflicting headers.

This change ensures that we're accounting for both lowercase and non-lowercase
default headers in Rails.

Author: obrie

lib/secure_headers/railtie.rb

@@ -22,9 +22,12 @@ class Railtie < Rails::Railtie
         ActiveSupport.on_load(:action_controller) do
           include SecureHeaders
 
-          unless Rails.application.config.action_dispatch.default_headers.nil?
-            conflicting_headers.each do |header|
-              Rails.application.config.action_dispatch.default_headers.delete(header)
+          default_headers = Rails.application.config.action_dispatch.default_headers
+          unless default_headers.nil?
+            default_headers.each_key do |header|
+              if conflicting_headers.include?(header.downcase)
+                default_headers.delete(header)
+              end
             end
           end
         end