Compare flags.cache to the string '0', not 0

[jeffposnick]

The logic in #245 is faulty, as flags is set via

const flags = args.parse(process.argv, { minimist })

and that will assign a string representation of all of the flag values.

This PR updates the code to handle -c 0 by doing the comparison against the string '0' instead of the number 0, to account for the fact that flags.cache is a string.

[jeffposnick]

Actually, my mistake. -c 0 should be working properly in master already.

I was thrown because of that flags.cache === 0 check, which I don't think is every going to be true... but the initial check of flags.cache will be true, since '0' is a true value, so I don't think that other clause will ever be triggered.

So maxAge=0 is being set, but that whole Cache-Control: no-cache introduced in https://github.com/zeit/serve/pull/245/files isn't being set. They have similar effects, though.