Fatal PHP error: Class 'CRM_Utils_String' not found when using CRM_Utils_Mail::send()

When attempting to run a custom script in the bin directory that includes CRM/Utils/Mail.php and makes use of CRM_Utils_Mail:send() the following error occurs:

PHP Fatal error: Class 'CRM_Utils_String' not found in ./CRM/Utils/Mail.php on line 87

Adding the following line just before the offending line 87 above fixes the issue.

require_once 'CRM/Utils/String.php';

This issue did not exist in at least <= 3.1.6, so it seems like a regression. The attached patch implement the above fix.