fix(main.js): resolve deprecation warning on Node.js 12 (#45)

Fix Node version 12 deprecated OutgoingMessage.prototype._headers

Co-authored-by: Raphael von der GrĂ¼n <raphinesse@gmail.com>
diff --git a/src/main.js b/src/main.js
index 69d4059..e651f12 100644
--- a/src/main.js
+++ b/src/main.js
@@ -33,7 +33,7 @@
         res.on('finish', function () {
             const color = this.statusCode === '404' ? chalk.red : chalk.green;
             let msg = `${color(this.statusCode)} ${this.req.originalUrl}`;
-            const encoding = this._headers && this._headers['content-encoding'];
+            const encoding = this.getHeader('content-encoding');
             if (encoding) {
                 msg += chalk.gray(` (${encoding})`);
             }