Honestly, trying out several combinations, using Esc before Space may be your quickest way. It also lets you "exit" when you're done adding all the classes you need,
e.g. class="btn<ESC><SPACE>btn-secondary<ESC><SPACE>alert<ESC><SPACE>alert-info<SPACE>" <-- this quote is skipped at the last space and you can add other attirbutes
Another option is to disable the auto-completion of attributes entirely. This can be done by going to Tools
> Options
> Text Editor
> HTML
> Advanced
, then set Insert attribute value quotes
to false
.