Add Support to Copy zero length Slice

[koota110]

res := []SampleArray{}
if err := copier.Copy(&SampleArray, sourceArray); err != nil {
	return nil, errors.Wrap(err, "failed to copy response")
}

if sourceArray is zero length array then res is nil !!

i want res is zero length array!!

[math-nao]

Could you please give a full test to run?

[rahulkhairwar]

@koota110 please look at this piece of code : https://play.golang.org/p/Ihqb0zTa8n0
I'm not able to reproduce the issue you've reported.
I think this ticket can be closed.
@jinzhu