Unreplace of assign_map should be to g:vimrplugin_assign_map

Behavior was not correct in the case that vimrplugin_assign_map is set
to something other than "_".  "Un"replacing vimrplugin_assign_map was
using "_" instead of the vimrplugin_assign_map string.
Merge pull request #130 from mdlerch/assign_map_fix

Unreplace of assign_map should be to g:vimrplugin_assign_map