silent Warning cast with PHP 5.3.3 (RHEL-6)

When running test suite on RHEL-6, with stock PHP 5.3.3, it fails with

     1) CacheTest::testDirectOverrideLegacy
     Failed asserting that exception of type "PHPUnit_Framework_Error_Warning" matches expected exception "Exception_Success".

     2) CacheTest::testDirectOverrideNew
     Failed asserting that exception of type "PHPUnit_Framework_Error_Warning" matches expected exception "Exception_Success".

After analysis, this is due to PHP warning in register method

     PHP Warning: Unknown class passed as parameter in ....SimplePie/Registry.php on line 116

This trivial fix simply silent this warning.