Added backpressure support to `staticCache()`

lib/middleware/staticCache.js

@@ -75,11 +75,21 @@ module.exports = function staticCache(options){
 
       // respond with cache
       res.writeHead(200, header);
-      for (var i = 1, len = hit.length; i < len; ++i) {
-        res.write(hit[i]);
+
+      // backpressure
+      function write(i) {
+        var buf = hit[i];
+        if (!buf) return res.end();
+        if (false === res.write(buf)) {
+          res.once('drain', function(){
+            write(++i);
+          });
+        } else {
+          write(++i);
+        }
       }
-      res.end();
-      return;
+
+      return write(1);
     }
 
     // cache static