api generation: PATCH

[Teelevision]

PATCH operations should update only the attributes that are provided and leave the rest untouched. Currently the generated api code does not provide any means for the called code to observe whether an attribute was set to its default value (should be updated) or whether it was omitted (should remain untouched).

Also think about whether it would be correct and whether we should support setting an attribute to null to reset it to its default value or delete it, and how we would pass along this info.