Pr/bugfix test request headers

Build request cookie objects from `request->headers`, rather than `request->env`.

As the request object is built, headers from the env are added to `request->headers`, however cookies were still being populated from the env.

When testing you may need to pass in extra headers into the request. Dancer2::Test allows for this updating the headers in the request, but any extra cookies were only accessible via `request->header`, not `request->cookies`.