null auto test cases now return 204 rather than JSON

[dansanduleac]

Before this PR

Auto-deserialize tests would always send the literal JSON value in the test to the client.

Afer this PR

If the value is null, then we respond with a 204 instead.

Added an receiveRawOptionalExample endpoint that returns an alias to a optional<integer> to exercise this:

curl -si http://127.0.0.1:8000/receiveRawOptionalExample/0
HTTP/1.1 204 No Content
access-control-allow-origin: *
content-length: 0
date: Fri, 31 Aug 2018 17:02:26 GMT

and sending it back as null is accepted:

curl -si http://127.0.0.1:8000/confirm/receiveRawOptionalExample/0 -H 'content-type: application/json' -d 'null'
HTTP/1.1 204 No Content
access-control-allow-origin: *
content-length: 0
date: Fri, 31 Aug 2018 17:04:41 GMT

Fixes #38

[qinfchen]

@iamdanfox, as per your comment in palantir/conjure-python#16, do we need to handle map, set, and list? Note that the spec.md only recommends 204 for empty containers, and 200 is fine as well.

[ferozco]

Looks good to me! Keen to see how this impacts our implementations