add support to multiple set-cookie headers

[oseau]

Hi,

Currently multiple set-cookie headers is folded into one header, eg:

'set-cookie': 'a=aValue; Max-Age=86400; Path=/w; HttpOnly; Secure; SameSite=None, b=bValue; Max-Age=86400; Path=/w; HttpOnly; Secure; SameSite=None',

However the correct header should be:

'set-cookie': [
    'a=aValue; Max-Age=86400; Path=/w; HttpOnly; Secure; SameSite=None',
    'b=bValue; Max-Age=86400; Path=/w; HttpOnly; Secure; SameSite=None'
  ],

I made a quick fix according to the info here https://developers.cloudflare.com/workers/runtime-apis/headers/#differences

Thanks for this awesome lib.

[oseau]

any updates? @kwhitley