fix: remove undocumented error.headers

Author: gr2m

lib/http-error.js

@@ -1,13 +1,14 @@
 module.exports = class HttpError extends Error {
-  constructor (message, code, headers) {
+  constructor (message, code) {
     super(message)
 
-    // Maintains proper stack trace
+    // Maintains proper stack trace (only available on V8)
+    /* istanbul ignore next */
     if (Error.captureStackTrace) {
       Error.captureStackTrace(this, this.constructor)
     }
+
     this.name = 'HttpError'
     this.code = code
-    this.headers = headers
   }
 }