Not all combinations of unicode/non-unicode parameters work for ljust,
center and rjust. Passing a unicode character to them as a parameter
when the string is ascii fails with an error.

This doctest fails in 3 places. Though I would expect it to be passing.

def doctest_strings():
    """

      >>> uni = u"a"
      >>> ascii = "a"

      >>> uni.center(5, ascii)
      u'aaaaa'

      >>> uni.center(5, uni)
      u'aaaaa'

      >>> ascii.center(5, ascii)
      'aaaaa'

      >>> ascii.center(5, uni)
      u'aaaaa'

      >>> uni.ljust(5, ascii)
      u'aaaaa'

      >>> uni.ljust(5, uni)
      u'aaaaa'

      >>> ascii.ljust(5, ascii)
      'aaaaa'

      >>> ascii.ljust(5, uni)
      u'aaaaa'

      >>> uni.rjust(5, ascii)
      u'aaaaa'

      >>> uni.rjust(5, uni)
      u'aaaaa'

      >>> ascii.rjust(5, ascii)
      'aaaaa'

      >>> ascii.rjust(5, uni)
      u'aaaaa'

    """