Fix PCT Test

[mtughan]

HtmlUnit does not appear to support the [key] syntax for setting property IDs in ECMAScript objects. To work around this, create an object specifically for the request headers and set the property using the standard syntax instead of setting it during object creation.

Fixes an issue introduced by #491 and will allow jenkinsci/bom#3524 to be merged.

Testing done

Tested locally using the PCT on the weekly line. Was failing before this change and succeeds afterwards.

Submitter checklist

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• [x] Ensure that the pull request title represents the desired changelog entry
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• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[mtughan]

@basil and @timja: not specifically related to this fix, but Linux-based executors seem to be stuck on https://ci.jenkins.io/. Can someone investigate that?