Per the JSON-HAL specification, a link relation can reference a
single link or an array of links. By default, two or more links with
the same relation will be treated as an array of links. The $singular
flag will force links with the same relation to be overwritten. The
$plural flag will force links with only one relation to be treated
as an array of links. The $plural flag has no effect if $singular
is set to true.