Describe the feature

Background: jshttp/cookie#60 (found it in an issue reported by @atinux)

Incoming cookie value can have the same name repeated and we pick the first.

We can allow them in two possible ways:

• { sameKeysPolicy: 'last' } as proposed in Reading duplicated cookies jshttp/cookie#60
• { multiple: true } that makes return type Record<string, string | string[]>
• { multiple: true } that concatinates multiple values with ,