Fixed ToMayaCurveConverter for cubic non periodic curves.

Cubic non periodic curves in Maya have implicit duplicate start/end CVs. Our FromMaya converter adds those extra CVs, so we need to remove the first 2 and last 2 when converting back to Maya. 
Conversion fails for cubic non periodic curves if they don't have at least 8 CVs.
Knots need to be tripled at the ends for cubic curves to get the curve start/end at the CVs.
Updated ToMayaCurveConverter test accordingly.