Don't send the empty trailers when we've sent a 200 OK to a CONNECT request

[Lukasa]

As shown in apple/swift-nio-ssl#539, when responding to a CONNECT request, if you send a .end then we will happily send a chunk-encoded empty blob. That is, the code below:

    func channelRead(context: ChannelHandlerContext, data: NIOAny) {
        let part = unwrapInboundIn(data)
        guard case let .head(request) = part, request.method == .CONNECT else {
            context.fireChannelRead(data)
            return
        }

        connectHost = request.uri.components(separatedBy: ":").first

        var responseHead = HTTPResponseHead(version: request.version, status: .ok)
        responseHead.headers.add(name: "Connection", value: "Established")
        context.write(self.wrapOutboundOut(.head(responseHead)), promise: nil)
        context.writeAndFlush(self.wrapOutboundOut(.end(nil))).whenComplete { _ in
            self.upgradeToTLS(context: context)
        }
    }

is incorrect. The .end should not be sent.

We can do better than this, and refuse to send anything in this context.