application controller: fix errors in normalize_search.

Fix exceptions in `normalize_search` on e.g. `https://danbooru.donmai.us/users?search=blah`.
Caused when the `search` param is not a hash.

app/controllers/application_controller.rb

@@ -189,8 +189,8 @@ class ApplicationController < ActionController::Base
   # /tags?search[name]=touhou&search[category]=&search[order]=
   # => /tags?search[name]=touhou
   def normalize_search
-    return unless request.get?
+    return unless request.get? && params[:action] == "index"
-    params[:search] ||= ActionController::Parameters.new
+    params[:search] = search_params
 
     deep_reject_blank = lambda do |hash|
       hash.reject { |k, v| v.blank? || (v.is_a?(Hash) && deep_reject_blank.call(v).blank?) }
@@ -204,7 +204,9 @@ class ApplicationController < ActionController::Base
   end
 
   def search_params
-    params.fetch(:search, {}).permit!
+    search_params = params.fetch(:search, {})
+    search_params = ActionController::Parameters.new unless search_params.is_a?(ActionController::Parameters)
+    search_params.permit!
   end
 
   def set_safe_mode

test/functional/application_controller_test.rb

@@ -35,5 +35,10 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
         assert_response 410
       end
     end
+
+    should "normalize search params" do
+      get tags_path, params: { search: { name: "bkub", post_count: "" } }
+      assert_redirected_to tags_path(search: { name: "bkub" })
+    end
   end
 end