Fix :Rmigration with space in directory name

Closes tpope#164.

Author: tpope

autoload/rails.vim

@@ -2513,10 +2513,10 @@ function! s:app_migration(file) dict
     let glob = '*'.rails#underscore(arg).'*rb'
   endif
   let files = split(glob(self.path('db/migrate/').glob),"\n")
-  if arg == ''
+  call map(files,'strpart(v:val,1+strlen(self.path()))')
+  if arg ==# ''
     return get(files,-1,'')
   endif
-  call map(files,'strpart(v:val,1+strlen(self.path()))')
   let keep = get(files,0,'')
   if glob =~# '^\*.*\*rb'
     let pattern = glob[1:-4]