Use appropriate type in generators test

This fixes the following thor's warning.

```
Expected string default value for '--generate'; got false (boolean)
```

railties/test/generators_test.rb

@@ -200,7 +200,7 @@ class GeneratorsTest < Rails::Generators::TestCase
 
     self.class.class_eval(<<-end_eval, __FILE__, __LINE__ + 1)
       class WithOptionsGenerator < Rails::Generators::Base
-        class_option :generate, :default => true
+        class_option :generate, default: true, type: :boolean
       end
     end_eval