units: Multiplying a string by a unit has strange behavior

I think I know what's causing this (the input to `Quantity` is iterable if it's a string?) but I'm not going to fix it until #929 is accepted/rejected...

Check it out:

    >>> "15"*u.Myr
    <Quantity 15 Myr>
    >>> ("15"*u.Myr) == 15.*u.Myr
    False
    >>> q = "15"*u.Myr
    >>> q.value
    array('15',
          dtype='|S2')