Handle the Encoding being a dictionary in PartialEvaluator_preEvaluateFont (bug 1157493)

*This is a regression from PR 4423.*

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1157493.

src/core/obj.js

@@ -192,6 +192,10 @@ var Dict = (function DictClosure() {
       return all;
     },
 
+    getKeys: function Dict_getKeys() {
+      return Object.keys(this.map);
+    },
+
     set: function Dict_set(key, value) {
       this.map[key] = value;
     },