Fixed an issue with mixed-case headers not being matched correctly in CORS

[giovannicalo]

After the CORS rework, headers seem to be matched in a case-sensitive way.
Therefore, even if a header is allowed...

cors: {
    additionalHeaders: [
        "X-Requested-With"
    ]
}

... and the browser sends it (with a different case)...

Access-Control-Request-Headers: x-requested-with

... it won't match, thus causing the CORS preflight request to fail.

This fixes it.

The horrible blank lines are there to please the linter.
Feel free to rewrite it differently.

[hueniverse]

Thanks. Did it a bit differently to make runtime faster.

[lock]

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