Previously LinkHeaders would delimit multiple links with \n alone
like so:
Link: <http://example.org/a>; rel="next"
<http://example.org/b>; rel="prev"
However, comma + newline is used as the delimiter in the
specification's (https://tools.ietf.org/html/rfc5988#section-5.5)
examples and, additionally, it states:
> Note that extension relation types are REQUIRED to be absolute
> URIs in Link headers, and MUST be quoted if they contain a
> semicolon (";") or comma (",") (as these characters are used as
> delimiters in the header itself).
In comma + newline appears to be the standard in practice (e.g.
GitHub's API). This changes LinkHeaders to use comma + newline as
the delimieter between multiple links, e.g:
Link: <http://example.org/a>; rel="next",
<http://example.org/b>; rel="prev"
