Disable default features on Rocket

[jhpratt]

This change should allow using rocket_cors with Rocket's async branch, which has (indirectly) updated its dependency on ring. Since rocket_cors doesn't use cookies anywhere, this change effectively does nothing but increase compatibility.

[lawliet89]

LGTM but I am not sure if this will cause issues when someone has

• Rocket in their dependency tree with default features enabled
• and passes a type constructed with their copy of Rocket into this library which only understands Rocket types with default features enabled.

[jhpratt]

I'm fairly certain cargo takes the greatest common denominator, so it would enable the feature if necessary, but leave if out otherwise.

[lawliet89]

The build will fail on an older nightly version. Will merge it once the tests pass on latest nightly.

I will go fix that... some day.

[jhpratt]

Ok. I tested it locally before creating this PR and everything passed. That's on a recent (~1 week old) nightly.

[lawliet89]

Failed due to new clippy lints. I will fix them some time later.