What Content-Type should a 204 No Response use?

You have Content-Type when you have content and even then it's optional:

Any HTTP/1.1 message containing an entity-body SHOULD include a Content-Type header field defining the media type of that body.

(from RFC2616, §7.2.1, please note the use of SHOULD).

Since there is no content, then there is no reason to specify a Content-Type header.

Well, there's a little more to the story though, and it involves when the real-world (browsers) meet the theoretical world (specifications). Seems Firefox (ver 38 as I write this) has a bug where the browser attempts to parse the content as xml if there's no content-type header, EVEN WHEN the response code is 204 (no content). See https://bugzilla.mozilla.org/show_bug.cgi?id=521301

So, while a content type header doesn't make much sense when there's no content (and there MUST NOT be content on a 204) it seems to make sense to return some header anyway. And I don't see where that would be a violation of the spec.