Send preload links via HTTP Link headers in addition to LINK tags

[AhmarZaidi]

Summary

Fixes #1321

Relevant technical choices

• Added a new method get_headers() to OD_Preload_Link_Collection to construct the Link HTTP response headers for preloading resources.
• Updated od_optimize_template_output_buffer to call headers() if there are preload links available and headers have not been sent already.
• Sent HTTP Link headers separately (above appending the link tags) to prevent Cannot modify header information - headers already sent error due to output before sending headers.

[AhmarZaidi]

Please add type label and milestone.

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Co-authored-by: AhmarZaidi <ahmarzaidi@git.wordpress.org>
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[westonruter]

Thank you! Great start.

[westonruter]

Oh, we'll also need to add tests for this new method.

[westonruter]

Getting close!

[AhmarZaidi]

Oh, we'll also need to add tests for this new method.

Had some doubts regarding tests implementation:

1. I believe get_html() is tested by using the output of od_optimize_template_output_buffer() since the walker adds links to the markup and compares it with the data from data provider.
   Now, how would I test for get_response_header() since they are added to the http response and not the markup.
   I explored xdebug_get_headers() but that doesn't seem to work. Could separately test get_response_header() function's output but I don't think that's the right way.

2. If we implement it like this in optimization.php:

// Send any preload links in a Link response header and in a LINK tag injected at the end of the HEAD.
if ( count( $preload_links ) > 0 ) {
	if ( ! headers_sent() ) {
		header( $preload_links->get_response_header() );
	}
	$walker->append_head_html( $preload_links->get_html() );
}

Then headers() won't be called while testing because headers_sent() would be true due to prior output, so how would we test it?

[westonruter]

@AhmarZaidi Yeah, testing the output of header() is a pain. I don't think we can practically test it being output when calling od_optimize_template_output_buffer(). I think it's fine to just test that get_response_header() method in isolation.

[westonruter]

The imagesrcset, imagesizes, and media attribute values here seem wrong being URL-encoded. Do these work being encoded like this?

[westonruter]

Indeed. I found it doesn't work, for imagesrcset at least. I tried sending that Link header on a Glitch and I saw a network request attempted for:

https://alert-sustaining-acrylic.glitch.me/https%3A%2F%2Fexample.com%2Ffoo-480w.jpg%20480w%2C%20https%3A%2F%2Fexample.com%2Ffoo-800w.jpg%20800w

[westonruter]

I think I got this. See d1cd8d4

[westonruter]

Great work!