Build prototype with Object.assign to allow minification (#1475)

lib/resize.js

@@ -469,13 +469,11 @@ function withoutEnlargement (withoutEnlargement) {
  * @private
  */
 module.exports = function (Sharp) {
-  [
+  Object.assign(Sharp.prototype, {
     resize,
     extend,
     extract,
     trim
-  ].forEach(function (f) {
-    Sharp.prototype[f.name] = f;
   });
   // Class attributes
   Sharp.gravity = gravity;