fix: do not discard already present set-cookie header

[rclement]

This PR aims at fixing the current behavior of nuxt-i18n when setting the detected language cookie.
If someone were to set a cookie within a Nuxt server-middleware (e.g. a CSRF token cookie), it will be discarded only the first time nuxt-i18n is setting the language redirection cookie.

The root cause comes from using res.setHeader('Set-Cookie', redirectCookie): it no longer implicitly append the cookie value to the current list of Set-Cookie values in Express > 4, as explained here: https://github.com/expressjs/express/wiki/Migrating-from-3.x-to-4.x#ressetheaderset-cookie-val

To keep compatibility of this middleware with requests/responses from Express/Connect/etc., using the cookies package instead of cookie seems like a sensible choice to me, and fixes nicely the issue.

If I missed something while refactoring the code, please let me know so we can find the most elegant and robust solution.

[codecov]

Codecov Report

Merging #327 into master will not change coverage.
The diff coverage is n/a.

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##           master   #327   +/-   ##
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  Coverage     100%   100%           
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  Files           2      2           
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  Hits            6      6

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[paulgv]

LGTM! Thank you @rclement !

[rchl]

Another way to fix that would be to get existing cookies and modify the array. Maybe that' better option than introducing this heavier dependency