Fixed MultipartBeam formatting corner case.

Consider this example:

    >>> container = Container("c'8 r4 c'8")
    >>> beam = spannertools.MultipartBeam()
    >>> attach(beam, container)

OLD:

    >>> f(container)
    {
        c'8 ]
        r4
        c'8
    }

NEW:

    >>> f(container)
    {
        c'8
        r4
        c'8
    }

Note that the dangling close-beam no longer appears.

The RatioTaleaRhythmMaker doctest examples should now be completely correct.