Adding a small fix to the Same() func

This is a little tricky, but when a diff contains a computed list or
set that can only be interpolated after the apply command has created
the dependant resources, it could turn out that the result is actually
the same as the existing state which would remove the key from the diff
Merge pull request #681 from svanharmelen/f-core-fix-diffing-diffs

core: adding a small fix to the Same() func (diffing diffs)