fix underscored method for single letter words

Author: az7arul

test/strings.js

@@ -436,6 +436,7 @@ $(document).ready(function() {
 
   test('String: underscored', function(){
     equal(_('oneAtATime').underscored(), 'one_at_a_time');
+    equal(_('oneAtATime AnotherWordAtATime').underscored(), 'one_at_a_time_another_word_at_a_time');
     equal(_('the-underscored-string-method').underscored(), 'the_underscored_string_method');
     equal(_('theUnderscoredStringMethod').underscored(), 'the_underscored_string_method');
     equal(_('TheUnderscoredStringMethod').underscored(), 'the_underscored_string_method');

underscored.js

@@ -1,5 +1,5 @@
 var trim = require('./trim');
 
 module.exports = function underscored(str) {
-  return trim(str).replace(/([a-z\d])([A-Z]+)/g, '$1_$2').replace(/[-\s]+/g, '_').toLowerCase();
+  return trim(str).replace(/([a-z\d])([A-Z]+)/g, '$1_$2').replace(/([A-Z\d]+)([A-Z][a-z])/g,'$1_$2').replace(/[-\s]+/g, '_').toLowerCase();
 };