Extend res.links() to allow adding multiple links with the same rel

[Myhlamaeus]

Currently, res.links allows to only add a single link per rel per call. One can add multiple link with the same rel through calling res.links multiple times. #2575 would also allow to add multiple links with the same call. Both of these methods are, however, quite cumbersome when one wants to simply add multiple links with the same rel and without optional parameters.

A possible alternative would be to allow the following:

res.links({
  self: '/a',
  item: ['/a/0', '/a/1']
})

Note that this would currently result in Link: </a>; rel="self", </a/0,/a/1>; rel="item", which is incorrect - the path may contain commas, so /a/0,/a/1 is a single path. The expected result would be Link: </a>; rel="self", </a/0>; rel="item", </a/1>; rel="item".