support status code 204

[dotSlashLu]

I've seen status 204 (No content) returning from our Redmine 3.3.0.stable when a project was successfully deleted by redmine.project.delete().

[coveralls]

Changes Unknown when pulling 01c04c5 on dotSlashLu:master into ** on maxtepkeev:master**.

[dotSlashLu]

I checked Redmine's code, there are no sign why I got a 204. But I did find something related to this: Updating a version through rest API returns invalid JSON

I think maybe I'm using a modified version of Redmine. Anyway, a status of 204 is perfectly valid in such a situation in my opinion, since the project no longer exists. I think maybe we should loose the status code validation, it seems no harm to me to consider all 2xx codes as valid and successful return values.

[maxtepkeev]

Thanks for the PR. I don't think that adding all the remaining 2xx codes makes sense for now, but adding a 204 is definitely a good idea.