No need for `filetype off` unless `g:did_load_filetypes` exists

Reference: https://github.com/tpope/vim-pathogen/pull/26

plug.vim

@@ -203,7 +203,9 @@ function! plug#end()
   endif
   let lod = { 'ft': {}, 'map': {}, 'cmd': {} }
 
-  filetype off
+  if exists('g:did_load_filetypes')
+    filetype off
+  endif
   for name in g:plugs_order
     if !has_key(g:plugs, name)
       continue