Revert params.fetch to params.expect conversions in scaffold

The conversions have the unintended consequence of requiring
params when none are expected on the default generated templates.

Originally pointed out in rails/jbuilder#573.

railties/lib/rails/generators/rails/scaffold_controller/templates/api_controller.rb.tt

@@ -48,7 +48,7 @@ class <%= controller_class_name %>Controller < ApplicationController
     # Only allow a list of trusted parameters through.
     def <%= "#{singular_table_name}_params" %>
       <%- if attributes_names.empty? -%>
-      params.expect(<%= singular_table_name %>: {})
+      params.fetch(:<%= singular_table_name %>, {})
       <%- else -%>
       params.expect(<%= singular_table_name %>: [ <%= permitted_params %> ])
       <%- end -%>

railties/lib/rails/generators/rails/scaffold_controller/templates/controller.rb.tt

@@ -55,7 +55,7 @@ class <%= controller_class_name %>Controller < ApplicationController
     # Only allow a list of trusted parameters through.
     def <%= "#{singular_table_name}_params" %>
       <%- if attributes_names.empty? -%>
-      params.expect(<%= singular_table_name %>: {})
+      params.fetch(:<%= singular_table_name %>, {})
       <%- else -%>
       params.expect(<%= singular_table_name %>: [ <%= permitted_params %> ])
       <%- end -%>

railties/test/generators/scaffold_controller_generator_test.rb

@@ -67,7 +67,7 @@ def test_dont_use_require_or_permit_if_there_are_no_attributes
 
     assert_file "app/controllers/users_controller.rb" do |content|
       assert_match(/def user_params/, content)
-      assert_match(/params\.expect\(user: \{\}\)/, content)
+      assert_match(/params\.fetch\(:user, \{\}\)/, content)
     end
   end