http: replace finish() callback with arrow function

Take advantage of arrow function lexical `this` to avoid defining a
`self = this` var which was only used once.

Code relating to the `finish` event was split in to two areas of the
parent function. Gathered it together to clarify association within the
script.

Fixes: #7295
PR-URL: #7378
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_http_outgoing.js

@@ -546,14 +546,6 @@ OutgoingMessage.prototype.end = function(data, encoding, callback) {
     return false;
   }
 
-  var self = this;
-  function finish() {
-    self.emit('finish');
-  }
-
-  if (typeof callback === 'function')
-    this.once('finish', callback);
-
   if (!this._header) {
     if (data) {
       if (typeof data === 'string')
@@ -581,6 +573,13 @@ OutgoingMessage.prototype.end = function(data, encoding, callback) {
     this.write(data, encoding);
   }
 
+  if (typeof callback === 'function')
+    this.once('finish', callback);
+
+  const finish = () => {
+    this.emit('finish');
+  };
+
   if (this._hasBody && this.chunkedEncoding) {
     ret = this._send('0\r\n' + this._trailer + '\r\n', 'binary', finish);
   } else {