Fix TypeError in libpq.PQconnectdb with unicode

Guarentees conninfo arguement for PQconnectdb c call is of type str
Prior to this, dsn could be of type unicode which raises a TypeError

```python
­­­>>> type(self.dsn)
<type 'unicode'>
>>> libpq.PQconnectdb(self.dsn)
TypeError: initializer for ctype 'char *' must be a str or list or tuple, not unicode
```