Use Cookie.__init__ instead of settings keys as a dict [Fixes #254]

[Pirulax]

Goal

Fix issue #254

New Features/Changes

The functionality should've remained the same, it just fixes the deprecation warning that was emitted.
I'm unsure how this affects the min. supported version of Sanic, perhaps that should be updated too.

Open Questions and TODOs

• I think there might've been a bug with setting the secure key. The original code did a falsey check, but False is also a falsey value, so the code didn't set the secure key to False, but rather left it at whatever it was by default. I fixed this by doing an explicit None check. Now, the TODO here is to check if my assumption/fix is correct.
• Brief test - I tested the changes by logging into my site, and everything works as it used to, but this time with no deprecation warnings.

[Pirulax]

Okay, so in the meanwhile I discovered 2 bugs.

1. On localhost cookie_domain() is '' (empty string), which originally became cookie_domain=None, but with my code it (was) cookie_domain='', which didn't really work all that well :D

2. There's something up with max_age too, and I'm unsure if it's intentional or not: config.cookie_max_age() is 0, which, again, is a falsey value, thus it's not set. I'm unsure if it's meant to be set in this case, or is supposed to be converted to None instead.

I've now changed my code to work like it originally did, but (2) should be looked into by someone more familiar with the lib.