Fix node encodings lookup in string parser

This fixes #582

Author: dolphin278

lib/parsers/string.js

@@ -1,15 +1,15 @@
 var Iconv = require('iconv-lite');
 
-var NODE_ENCODING = [
-  'ascii',
-  'utf8',
-  'utf16le',
-  'ucs2',
-  'base64',
-  'latin1',
-  'binary',
-  'hex'
-];
+var NODE_ENCODING = {
+  'ascii': true,
+  'utf8': true,
+  'utf16le': true,
+  'ucs2': true,
+  'base64': true,
+  'latin1': true,
+  'binary': true,
+  'hex': true
+};
 
 exports.decode = function(buffer, encoding, options) {
   if (NODE_ENCODING[encoding]) {