DEV: Use response.parsed_body in specs (#9615)

Most of it was autofixed with rubocop-discourse 2.1.1.

spec/requests/admin/user_fields_controller_spec.rb

@@ -49,7 +49,7 @@ describe Admin::UserFieldsController do
       it "returns a list of user fields" do
         get "/admin/customize/user_fields.json"
         expect(response.status).to eq(200)
-        json = ::JSON.parse(response.body)
+        json = response.parsed_body
         expect(json['user_fields']).to be_present
       end
     end