plugin/msgpack: Support character constants like '\0'

runtime/autoload/msgpack.vim

@@ -665,11 +665,15 @@ function msgpack#eval(s, special_objs) abort
       call add(expr, ']}')
       let s = s[1:]
     elseif s[0] is# ''''
-      let char = matchstr(s, '\m\C^''\zs.\ze''')
+      let char = matchstr(s, '\v\C^\''\zs%(\\\d+|.)\ze\''')
       if empty(char)
         throw 'char-invalid:Invalid integer character literal format: ' . s
       endif
-      call add(expr, char2nr(char))
+      if char[0] is# '\'
+        call add(expr, +char[1:])
+      else
+        call add(expr, char2nr(char))
+      endif
       let s = s[len(char) + 2:]
     else
       throw 'unknown:Invalid non-space character: ' . s

test/functional/plugin/msgpack_spec.lua

@@ -652,6 +652,8 @@ describe('In autoload/msgpack.vim', function()
 
       eval_eq('integer', ('a'):byte(), '\'a\'')
       eval_eq('integer', 0xAB, '\'«\'')
+      eval_eq('integer', 0, '\'\\0\'')
+      eval_eq('integer', 10246567, '\'\\10246567\'')
     end)
 
     it('correctly loads constants', function()