Accept "nil" in char const* converter.
Accept "nil" in char const* converter.

This brings back the behavior we had in 0.8. I want to get rid of this,
but since there's currently no way to get the old behavior it will have
to wait until 1.0.

decimad/luabind-deboostified,rpavlik/luabind,alex85k/luabind,grafi-tt/luabind,galek/luabind-deboostified,Kazade/luabind-deboostified,alex85k/luabind,plsytj/luabind,Oberon00/luabind,Wohlhabend-Networks/luabind-deboostified,rpavlik/luabind,MrApplejuice/luabind,mdave88/luabind,luabind/luabind,Oberon00/luabind,rpavlik/luabind,mapbox/luabind,luabind/luabind,davidar/luabind,ltjax/luabind-deboostified,luabind/luabind,rpavlik/luabind,alex85k/luabind,luabind/luabind,alex85k/luabind,rpavlik/luabind,lonski/luabind