Happy to remove OPTIONS from the list of methods that gets a content-length where body is None, but do we also want to consider behaviour if it's the empty string? My feeling is that '' implies "present but empty" (so should have a content-length set to zero), whereas None implies "missing" (so should only have a content-length header set to zero if the method is expecting a body.
I've updated the patch with this logic, including ensuring proper explicit test coverage for the cases described. Docs updated too.