better handling of requests that die

proxy.js

@@ -139,8 +139,14 @@ Proxy.prototype.next_socket = function(cb) {
         return self.waiting.push(cb);
     }
 
+    var done_called = false;
     // put the socket back
     function done() {
+        if (done_called) {
+            throw new Error('done called multiple times');
+        }
+
+        done_called = true;
         if (!sock.destroyed) {
             debug('retuning socket');
             self.sockets.push(sock);