The finish_content method in wsgiref.handlers.BaseHandler class resets
Content-Length to "0" if there was no content returned by the
application, even if the application has set it already.

That makes impossible to implement things like HEAD requests in the
application, which should return the Content-Length without returning
actual content.

The desired behavior would be to respect the header set by the wsgi
application, using "0" as a default value instead.