RSpec 4 compatibility: override #respond_to? method

According to:

```
  config.mock_with :rspec do |mocks|
    # Prevents you from mocking or stubbing a method that does not exist on
    # a real object. This is generally recommended, and will default to
    # `true` in RSpec 4.
    mocks.verify_partial_doubles = true
  end
```

and [rspec-mocks](https://github.com/rspec/rspec-mocks/blob/a32ecea6d20931c141a0e39c5f52d09633f8d892/lib/rspec/mocks/method_reference.rb#L83-L95)