`RationalTime.__add__` always uses the higher rate

Currently when you have:
```python
a = RationalTime(0, 12)
b = RationalTime(10, 24)
c = a + b
d = b + a

c.rate == d.rate == 24
# True
```
Both `c` and `d` will be at rate 24, because the `__add__` method always produces a `RationalTime` in the higher of the two rates. This might be the correct behavior, or it could be unpredictable.

A potential alternative would use the order of the arguments to choose which rate to use in the product. I'm filing this issue to see if anyone objects to this behavior or if we should leave it the way it is.