added names and skips arrays erasing on enable call

[musikov]

Namespaces couldn't be disabled once enabled.
This PR fixed this issue.

test:

let debug = require('debug');

console.log(1, debug.enabled('test'));

debug.enable('test');
console.log(2, debug.enabled('test'));

debug.disable();
console.log(3, debug.enabled('test'));

expected print:

1 false
2 true
3 false

[coveralls]

Coverage increased (+0.5%) to 63.75% when pulling a82c04f on musikov:master into ac5ccae on visionmedia:master.