Don't expect encoding-indexes to return object with indexes & don't r…

…equire them if they are already present (#61) - Resolves #60

* Don't require encoding-indexes if they are already present
* Don't expect encoding-indexes to return object with indexes

lib/encoding.js

@@ -6,9 +6,9 @@
  * @fileoverview Global |this| required for resolving indexes in node.
  * @suppress {globalThis}
  */
-if (typeof module !== "undefined" && module.exports) {
-  this["encoding-indexes"] =
-    require("./encoding-indexes.js")["encoding-indexes"];
+if (typeof module !== "undefined" && module.exports &&
+    !this["encoding-indexes"]) {
+    require("./encoding-indexes.js");
 }
 
 (function(global) {