feat: allow setting args for variadic functions

[bergotorino]

The gomock's Call interface allows for setting a value to a specified argument, making it convinient to work with functions that return data through pointer arguments. However, this is not possible for variadic functions, the error is:

SetArg(1, ...) called for a method with 1 args...

The root cause is that the variadic argument in a form of ...interface{} is threated as single value instead of being treated as slice.

This commit changes this situation by making sure that the variadic arguments that are a slice are treated as slices. As a consequence it is now possible to set argument value for variadic functions.

See: https://github.com/golang/mock/issues/174

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[bergotorino]

@googlebot I signed it!

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[bergotorino]

Sure, will do. Thanks for review and sorry for missing your comments on time

[mvrahden]

@bergotorino are you still planning to do this?

[codyoss]

Closing due to lack of response.