The spline record is ok, but the rest of the DXF R14 file is wrong.
Writing R14 files is rather complex. You need to get the handles right,
add an OBJECT section, other tables in the table section and more. I
suggest you go for DXF R12, if possible.

But I added the needed sections by hand. The result is attached to this
mail, and it loads into AutoCAD. You'll notice that the ENTITIES section
is the same, so you got your SPLINE right.

Hope this helps

Daniel