Allow QueryString.add_param() to work with int values.

This seems to happen to me all the time, where higher level code assigns
integer values and then we get a traceback.  Seems harmless to handle integer
querystring values.

This will allow me to avoid the following hack in my code:

```python
# HACK since URLObject doesn't like ints in query values.
data = {k: str(v) if isinstance(v, int) else v
        for k, v in data.items()}
url = url.set_query_params(**data)
```