*sp overrides takes no effect

I trying to override some underline colors, let us say:
```
let g:jellybeans_overrides = {
      \  'SpellCap'  : { '256cterm': 'undercurl', '256ctermbg': 'None',
      \                  'guisp': 'Red, '256ctermsp': 'Red' },
      \}
```
But I still have:
```
:hi SpellCap
SpellCap       xxx cterm=underline gui=underline guibg=#0000df guisp=Blue
```