Add tests for including an arrayref of headers to dancer_response

Ensuring there are tests to match the documentation. Note that extra
cookie headers are currently not being included in the request object cookies
hash. Ref #500.
Build request cookie objects from request->headers, rather than request->env.

As the request object is built, headers from the env are added to the headers
object, however cookies were still being populated from the env. However, when
testing you may need to pass in extra headers in 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.

Building the cookie objects from request->headers resolves this and closes #500.
reflect changes
Merge branch 'pr/bugfix_test_request_headers'