Fix bug for close connection response with no body.

cow should add content-length: 0 header to indicate the end of close
connection responses with no body, add chunked encoding for those has body.

Safari will wait for body when cow incorrectly adds the chunked header.
As Safari limits number of concurrent connections to the proxy, it will
block on such erroneous responses.

http.go

@@ -602,8 +602,20 @@ func parseResponse(sv *serverConn, r *Request, rp *Response) (err error) {
 	// Connection close, no content length specification
 	// Use chunked encoding to pass content back to client
 	if !rp.ConnectionKeepAlive && !rp.Chunking && rp.ContLen == -1 {
-		rp.raw.WriteString("Transfer-Encoding: chunked\r\n")
+		if rp.hasBody(r.Method) {
+			debug.Println("add chunked encoding to close connection response", r, rp)
+			rp.raw.WriteString("Transfer-Encoding: chunked\r\n")
+		} else {
+			debug.Println("add content-length 0 to close connection response", r, rp)
+			rp.raw.WriteString("Content-Length: 0\r\n")
+		}
 	}
+	// Check for invalid response
+	if !rp.hasBody(r.Method) && (rp.Chunking || rp.ContLen != -1) {
+		errl.Printf("response has no body, but with chunked/content-length set\n%s",
+			rp.Verbose())
+	}
+
 	// Whether COW should respond with keep-alive depends on client request,
 	// not server response.
 	if r.ConnectionKeepAlive {