Avoid `any_instance_of` in controller scaffold

The [rspec-mocks README](https://github.com/rspec/rspec-mocks#settings-mocks-or-stubs-on-any-instance-of-a-class) advises against using `allow_any_instance_of` and `expect_any_instance of`, so I think these should be avoided in the generated controller scaffold.

This requires changing some `be_a_new` expectations to equality checks, but I don't think this is a problem.