Request headers should be also represented as an array

[karol-maciaszek]

Chore summary
Currently the type for IHttpRequest.headers is just string. This is good because even if a header of the particular name appears several times inside the request, they will be concatenated using , (see https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2).

However, this is not true for Set-Cookie which is not concatenated due to its internal format. Specifically, the expires can contain , already.
Example: Set-Cookie: id=a3fWa; Expires=Wed, 21 Oct 2015 07:28:00 GMT

The goal of this task is to make headers a string | string[] and update all the dependent code.

Tasks

• Find and asses dependencies
• Update type in http.ts
• Update dependent projects (might require separate tasks if too complex)

Additional context
Based on discussion here: https://github.com/stoplightio/platform-internal/pull/1279/files#r391020707