Support ordering w/ `NULL LAST`

[marks]

Bug: When I try to order with value desc null last soda-js converts it to value desc null last asc

I think this is because of this code snippet: https://github.com/socrata/soda-js/blob/5fad1c65d7a44deb921adaa308b6e0b0555c1a4a/src/soda-js.coffee#L65

[chrismetcalf]

@marks Whoa, I didn't know you could do that with SoQL. This I will have to try out!