Add Object.assign polyfill

src/polyfills.js

@@ -27,3 +27,6 @@ if (typeof String.prototype.repeat === 'undefined') {
 if (typeof Symbol === 'undefined') {
   require('core-js/fn/symbol');
 }
+if (typeof Object.assign === 'undefined') {
+  require('core-js/fn/object/assign');
+}

src/worker/worker.js

@@ -86,7 +86,9 @@ self.onmessage = function(event) {
  * @param  {Object} config   The openpgp configuration
  */
 function configure(config) {
-  Object.assign(openpgp.config, config);
+  Object.keys(config).forEach(function(key) {
+    openpgp.config[key] = config[key];
+  });
 }
 
 /**