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>

Author: originalfoo

lib/_http_outgoing.js

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