A server MAY send a Content-Length header field in a 304 (Not
   Modified) response to a conditional GET request (Section 4.1 of
   [RFC7232]); a server MUST NOT send Content-Length in such a response
   unless its field-value equals the decimal number of octets that would
   have been sent in the payload body of a 200 (OK) response to the same
   request.

   A server MUST NOT send a Content-Length header field in any response
   with a status code of 1xx (Informational) or 204 (No Content).  A
   server MUST NOT send a Content-Length header field in any 2xx
   (Successful) response to a CONNECT request (Section 4.3.6 of
   [RFC7231]).