BUG: Fixed handling of non-list value_vars in melt
The `value_vars` argument of `melt` is now cast to `list` like the `id_vars` argument. This works for tuples, lists, ndarrays, and scalars.

 - [x] closes #15348 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry
