[SQL] Python JsonRDD UTF8 Encoding Fix

Only encode unicode objects to UTF-8, and not strings

Why do we need the `isinstance` here?  In `saveAsTextFile`, we just unconditionally encode strings as UTF-8:

```python
        def func(split, iterator):
            for x in iterator:
                if not isinstance(x, basestring):
                    x = unicode(x)
                yield x.encode("utf-8")
        keyed = self.mapPartitionsWithIndex(func)
        keyed._bypass_serializer = True
```

Is there a bug in this `saveAsTextFile` code?