Fix indent plugin for vimspec

'indentexpr` may be changed by user or other plugins.

indent/vimspec.vim

@@ -10,19 +10,19 @@ runtime! indent/vim.vim
 
 let b:did_indent = 1
 
-setlocal indentexpr=GetVimspecIndent()
+let &l:indentexpr = 'GetVimspecIndent(' . string(&l:indentexpr) . ')'
 setlocal indentkeys+==End
 
 if exists('*GetVimspecIndent')
   finish
 endif
 
-function GetVimspecIndent() abort
+function GetVimspecIndent(orig_indentexpr) abort
   try
     " Old Vim's indent plugin has a bug that uses =~
     let ignorecase_save = &ignorecase
     set noignorecase
-    let indent = GetVimIndent()
+    let indent = eval(a:orig_indentexpr)
   finally
     let &ignorecase = ignorecase_save
   endtry