escape spaces in s:rtp_add_defaults()

Unescaped spaces in home directory path (i.e. ".../Macintosh HD/Users/...") cause errors like these at startup:
```
Error detected while processing function vundle#config#bundle..<SNR>4_rtp_add_defaults:
line    6:
E518: Unknown option: HD/Users/Yukun/.vim/after
line    8:
E518: Unknown option: HD/Users/Yukun/.vim/after
line    6:
E518: Unknown option: HD/Users/Yukun/.vim
line   10:
E518: Unknown option: HD/Users/Yukun/.vim
line    6:
E518: Unknown option: HD/Users/Yukun/.vim/after
line    8:
E518: Unknown option: HD/Users/Yukun/.vim/after
```