Fixed an issue about urlize function

When URL startswith 'https://', the return is wrong.
Before:

    >>> urlize('https://github.com/')
    u'<a href="http://https://github.com/">https://github.com/</a>'
After:

    >>> urlize('https://github.com/')
    u'<a href="https://github.com/">https://github.com/</a>'