Serializing absent optional as empty body should omit content-type

[dansanduleac]

Identified in https://github.com/palantir/conjure-verification/issues/45

The wire spec states that all endpoints defining a body parameter must set one of two content-type headers:

Content-Type header - For Conjure endpoints that define a body argument, a Content-Type header MUST be added. If the body is of type binary, the content-type application/octet-stream MUST be used. Otherwise, clients MUST send Content-Type: application/json.

However, a bit further up under "Body parameter" it suggests (even recommends) that clients should send an empty body in the case of an absent optional body parameter, which doesn't seem to be valid JSON:

[when] the de-aliased argument is type optional<T> and the value is not present: it is RECOMMENDED to send an empty request body, although clients MAY alternatively send the JSON value null

Since we probably have to support this use case, we should say that content-type MUST NOT be set when sending an absent optional body as an empty body.

[iamdanfox]

One further argument in support of this wire-spec change is that Chrome actually strips the Content-Type header if a request body is empty!

[carterkozak]

If there's no content it cannot possibly have a type.

Except... if we send a binary request, binary data is allowed to be zero-bytes, and we have no way to differentiate between absent and present but zero bytes where responses can specify 200/204 status, which itself isn't quite right because no content should always use the 204 status... Additional musings in #186