Fix mock error by following error instructions

Fixes the following error:

```
[WARNING] Could not load generator
"generators/test_unit/decorator_generator". Error: Rails::Generators
received :require with unexpected arguments
  expected: ("application_decorator")
       got: ("generators/test_unit/decorator_generator")
 Please stub a default value first if message might be received with
other args as well
```

spec/generators/decorator/decorator_generator_spec.rb

@@ -40,6 +40,7 @@ describe Rails::Generators::DecoratorGenerator do
 
       context "with an ApplicationDecorator" do
         before do
+          allow_any_instance_of(Object).to receive(:require)
           allow_any_instance_of(Object).to receive(:require).with("application_decorator").and_return(
             stub_const "ApplicationDecorator", Class.new
           )