Ensure requests use cors mode on browsers

[Fryuni]

Summary

Change ContentFetcher and Evaluator to ensure cors request mode when running on a browser. This is necessary for requests to include the Origin header¹ (and therefore participate correctly on the CORS protocol) without relying on the security configurations of the surrounding script/page.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Footnotes

1. https://fetch.spec.whatwg.org/#origin-header ↩

[codeclimate]

Code Climate has analyzed commit 0ee85e9 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 99.9% (0.0% change).

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