Fixing issue of passing varargs and string to Sprintf. Closes #619

[devdinu]

Build fails with recent go version in mock package.

./mock.go:506: missing ... in args forwarded to printf-like function
./mock.go:523: missing ... in args forwarded to printf-like function

This fixes #619.

[georgelesica-wf]

Looks good to me. I ran into this and while this seems like a strange fix, it works.

[devdinu]

The tests were failing for all PRs before this fix, and recent PR's CI tests succeeds. So this must be specific to some tip version of go. We could drop this PR if its non mandated by go changes in release. We can retrigger the CI for the failed PR to verify.

[georgelesica-wf]

Good call, let's leave it open but put it on hold for now until we're sure we don't need it. Thanks.