setdashes bug

It is pretty standard. See e.g. the PostScript Language

    > Reference Manual. And, how else to set a solid line with
    > set_dashes?

I didn't know that about postscript. I can easily implement it if it
is "the right thing to do (tm)". But it never occurred to me to
implement a solid line with set_dashes. I would tend to do


Since it is part of postscript, I'll make it the default for mpl too.