Thanks again for valuable comments.

I agree that there could be a case where send_error() gets used for both HEAD and GET, in which case we could send content-length field as an optional field. However, if send_error() is not used for both HEAD and GET, then the body will likely be different and RFC specifies that we must not send content-length field with different values so I have a slight preference towards not sending it for HEAD request. 

If you are OK with leaving content-length out for HEAD, I'd like to proceed with creating a small patch to reflect your comments in code review and also start creating a patches for 2.7 and 3.5.