[IronPython] re.sub at the start of the string

[Sanghyeon Seo]

import re
print re.sub('^', 'x', 'abc')

CPython prints xabc, IronPython prints just abc.

For FePy users: you can use FEPY_OPTIONS=regex to avoid this bug for
the time being.

-- 
Seo Sanghyeon