fix #19, add more reliable type check

Author: Hanks10100

lib/util.js

@@ -2,7 +2,7 @@
  * rules:
  * - abc-def -> abcDef
  * - -abc-def -> AbcDef
- * 
+ *
  * @param  {string} value
  * @return {string}
  */
@@ -16,12 +16,15 @@ exports.hyphenedToCamelCase = function hyphenedToCamelCase(value) {
  * rules:
  * - abcDef -> abc-def
  * - AbcDef -> -abc-def
- * 
+ *
  * @param  {string} value
  * @return {string}
  */
 exports.camelCaseToHyphened = function camelCaseToHyphened(value) {
   return value.replace(/([A-Z])/g, function(s, m) {
-    return '-' + m.toLowerCase()
+    if (typeof m === 'string') {
+      return '-' + m.toLowerCase()
+    }
+    return m
   })
 }