vararg "_" in interface is mocked incorrectly

[TvdW]

The docker API client code contains an interesting interface:

...
	ImageInspect(ctx context.Context, image string, _ ...ImageInspectOption) (image.InspectResponse, error)
	ImageHistory(ctx context.Context, image string, _ ...ImageHistoryOption) ([]image.HistoryResponseItem, error)
	ImageLoad(ctx context.Context, input io.Reader, _ ...ImageLoadOption) (image.LoadResponse, error)
	ImageSave(ctx context.Context, images []string, _ ...ImageSaveOption) (io.ReadCloser, error)
...

While these are accepted on an interface, they cannot be named _ on the implementation.

Actual behavior

The resulting mock cannot be used

Expected behavior

It should be usable (_ renamed to arg%d)

To Reproduce

1. Attempt to mock the Docker API

Additional Information

Running code at HEAD