vim-patch:8.1.1279: cannot set 'spellang' to "sr@latin"

Problem:    Cannot set 'spellang' to "sr@latin". (Bojan Stipic)
Solution:   Allow using '@' in 'spellang'. (closes vim/vim#4342)
9a061cb78c

src/nvim/option.c

@@ -2568,7 +2568,7 @@ static bool valid_filetype(const char_u *val)
 bool valid_spellang(const char_u *val)
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  return valid_name(val, ".-_,");
+  return valid_name(val, ".-_,@");
 }
 
 /// Return true if "val" is a valid 'spellfile' value.