Set "link" HTTP header for canonical URL and shortlink tags

Drupal core sets all link elements as HTTP headers, too. For example, if a page has the following in the element:

<link rel="canonical" href="/foo">
<link rel="shortlink" href="/node/1">

Then it might also have the following in the HTTP response headers:

Link: </foo>; rel="canonical", </node/1>; rel="shortlink"

This is done in `HtmlResponseAttachmentsProcessor.php`: http://cgit.drupalcode.org/drupal/tree/core/lib/Drupal/Core/Render/HtmlR... . That code takes every `html_head_link` item and adds it to _both_ the HTML head and the HTTP header.

But in Metatags, the 'canonical_url' and 'shortlink' items are removed from `$build['#attached']['html_head_link']` if they are set for nodes (see metatag_entity_view_alter()). It adds them to to the HTML head using its own methods I guess, and it doesn't set the HTTP header at the same time.

I think that Metatag should add these HTTP response header links just like core does.