wrong handling of multiple unicode literals when --six-unicode is used

[daira]

If one has:

    foo(u'.........'
        u'.........')

it gets converted to:

    foo(six.u('........')
        six.u('........'))

that obviously does not work, better would be:

    foo(six.u('.........'
              '.........'))

(moved from https://github.com/mitsuhiko/python-modernize/issues/9 opened by @ThomasWaldmann)

[daira]

The test looks correct now (i.e. it fails for the right reason).