Fix double fetching resulting from preload hint cache miss if using CDN

[rtsao]

Per https://developers.google.com/web/updates/2017/12/modulepreload preload hints must be using the same credentials mode as the script tags for there to be a cache hit.

This PR ensures that preload hints have the same crossorigin attribute as the script tags to prevent double fetching.

[codecov]

Codecov Report

Merging #314 into master will increase coverage by 0.21%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
+ Coverage    91.4%   91.62%   +0.21%     
==========================================
  Files          18       18              
  Lines         419      418       -1     
  Branches       83       81       -2     
==========================================
  Hits          383      383              
  Misses         22       22              
+ Partials       14       13       -1

Impacted Files	Coverage Δ
src/plugins/ssr.js	90.47% <88.88%> (+1.41%)	⬆️

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[rtsao]

!merge

[mlmorg]

Is it possible to write a test for this?

[rtsao]

Yeah, to test this I would:

• Produce a production build from a fixture app w/ fusion-cli
• Start a separate static asset server for client chunks on a different port
• Start the production build w/ CDN_URL env set to the static asset server
• Start puppeteer and assert on "request" events to ensure chunk requests aren't duplicated

A less good test might be to simply assert on ctx.body and ensure crossorigin attributes are the same for preload hints as well as scripts when CDN_URL is used. This test would be quicker to implement but I think has dubious value.

Either way, preload hints are only present in the legacy body template, which will be deleted in the next major version. The new template lives in fusion-cli, added in (fusionjs/fusion-cli#531), which doesn't have preload hints because they won't work with the future <script type="module"> and <script nomodule> loading strategy.

For the sake of expediency in getting a pre-release out, I verified this fix in a production app but didn't write a proper test since this codepath has already been replaced. An issue for adding a regression test: #317