Fix numpy_gexpr initialization

There was a confusion between universal references and R-value
references. Doing a raw copy is the best match I can find here.
On Thu, Oct 09, 2014 at 11:34:17PM -0700, pbrunet wrote:
> In pythran/tests/cases/periodic_bc.py:
> > @@ -0,0 +1,5 @@
> > +#pythran export periodic_bc(float [][][])
> > +#runas import numpy; r = numpy.arange(0., 27.).reshape((3,3,3)); periodic_bc
> (r)
> > +def periodic_bc(f):
> > +        f[:, 0, :] = f[:, -2, :]
> > +
> This test return nothing. Can you return f to be sure it does not "just
> compile" but have a correct result?
yes