Update util.py - fix ensure_tuple instance check
Update util.py - fix ensure_tuple instance check

doing isinstance(obj, Sequence) makes calls with keys of dict impossible
Example:
Returns 
```
d = {'a':'a', 'b':'b'}
table.upsert(values, d.keys())
```
`dict_keys(['a', 'b'])` is not of instance Sequence, but Iterable, therefore `ensure_tuple(dict_keys(['a', 'b']))` returns (dict_keys(['a', 'b']),)
which leads to invalid iteration.
pudo/dataset,saimn/dataset