PX facet labels don't respect labels kwarg

```python
px.scatter(tips, x="total_bill", y="tip", facet_row="time",  facet_col="day", 
          labels={"time": "THE TIME", "day": "THE DAY"})
```

![image](https://user-images.githubusercontent.com/203523/70057695-94e75180-15ab-11ea-8643-1579ceedbc03.png)

Reported in https://github.com/plotly/plotly_express/pull/164