Fix util.promisify for patched read function

Fixes #125

PR-URL: #165
Credit: @coreyfarrell
Close: #165
Reviewed-by: @isaacs

polyfills.js

@@ -115,20 +115,26 @@ function patch (fs) {
   }
 
   // if read() returns EAGAIN, then just try it again.
-  fs.read = (function (fs$read) { return function (fd, buffer, offset, length, position, callback_) {
-    var callback
-    if (callback_ && typeof callback_ === 'function') {
-      var eagCounter = 0
-      callback = function (er, _, __) {
-        if (er && er.code === 'EAGAIN' && eagCounter < 10) {
-          eagCounter ++
-          return fs$read.call(fs, fd, buffer, offset, length, position, callback)
+  fs.read = (function (fs$read) {
+    function read (fd, buffer, offset, length, position, callback_) {
+      var callback
+      if (callback_ && typeof callback_ === 'function') {
+        var eagCounter = 0
+        callback = function (er, _, __) {
+          if (er && er.code === 'EAGAIN' && eagCounter < 10) {
+            eagCounter ++
+            return fs$read.call(fs, fd, buffer, offset, length, position, callback)
+          }
+          callback_.apply(this, arguments)
         }
-        callback_.apply(this, arguments)
       }
+      return fs$read.call(fs, fd, buffer, offset, length, position, callback)
     }
-    return fs$read.call(fs, fd, buffer, offset, length, position, callback)
-  }})(fs.read)
+
+    // This ensures `util.promisify` works as it does for native `fs.read`.
+    read.__proto__ = fs$read
+    return read
+  })(fs.read)
 
   fs.readSync = (function (fs$readSync) { return function (fd, buffer, offset, length, position) {
     var eagCounter = 0