Don't set the cookie flag if it has a falsy value in the options object

[papandreou]

Hey, thanks for a nice lib!

I found it a bit unintuitive that a flag is set if I specify it with a falsy value in the options object, eg.:

hardtack.set('foo', 'bar', {
  secure: location.protocol === 'https:'
});

Fix it by ignoring flags passed as false, undefined, or null, but don't break maxAge: 0.

Sorry for blowing the size budget ;)

[coveralls]

Coverage remained the same at 100.0% when pulling db69f0a on papandreou:fix/falsyOptionValue into 853ae41 on alik0211:master.

[alik0211]

It's good PR, but I'm not merge.

Handling false, undefined, null is not a task for such a small library. Similar functionality was removed in #41 and I don't expect to ever return.

[papandreou]

I think that's too surprising, people will expect it to work like that. It sounds like you're knowingly creating a footgun. At least consider documenting it, then.