Print got and want with %+v format flag instead of %v by default

[kakuiho]

Requested feature A clear description of the desired feature and an example of
how it would be used.

When a matcher reports a failure, it prints the received (Got) vs the expected (Want) value.

       Got: &{123 456 789} (*foo.FooORM)
        Want: is equal to &{1234 456 789} (*foo.FooORM)

We don't know which field of foo.FooORM is mismatch.

Why the feature is needed A clear description of how this feature is not
served by existing functionality in gomock.

Better format should be

       Got: &{a:123 b:456 c:789} (*foo.FooORM)
       Want: is equal to &{a:1234 b:456 c:789} (*foo.FooORM)

It prints struct with their field names by default, user would easier to figure it out what is wrong(the field a value is mismatch).

(Optional) Proposed solution A clear description of a proposed method for
adding this feature to gomock.

Print struct with %+v format flag instead of %v in Got and Want by default.

mock/gomock/string.go

Line 18 in d97cf0d

return fmt.Sprintf("%v", x)