fix iterable serialization

Iterables are not serialized correctly, as demonstrated by the code below:

tollsHighways = Directions().directions('paris', 'berlin', avoid=('tolls', 'highways'))[0]['legs'][0]['duration']
highwaysTolls = Directions().directions('paris', 'berlin', avoid=('highways', 'tolls'))[0]['legs'][0]['duration']
assert tollsHighways == highwaysTolls

These changes fix that
I want to leave the test here to show that google registers correctly the iterable params