Change js callout to add a response header instead of removing one

[danistrebel]

Description

What's changed, or what was fixed?

• Change sample and test case to add instead of remove headers

Fixes: #558

Housekeeping

(please check all that apply [x], do not edit the text)

• [x] I have run all the tests locally and they all pass.
• [x] I have followed the relevant style guide for my changes.
• [ ] PR requires full pipeline run (Run for changes only by default).

CC: @apigee-devrel-reviewers

[danistrebel]

/gcbrun

[apigee-devrel-bot]

Pipeline Report

Pipeline	Result	Elapsed Wall Time
references/js-callout	pass	133s
TOTAL PIPELINE	pass	133s

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Commit version: c14ef63

[danistrebel]

/gcbrun

[apigee-devrel-bot]

Pipeline Report

Pipeline	Result	Elapsed Wall Time
references/js-callout	pass	117s
TOTAL PIPELINE	pass	117s

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Commit version: 1a1e8ba

[danistrebel]

/gcbrun

[apigee-devrel-bot]

Pipeline Report

Pipeline	Result	Elapsed Wall Time
references/js-callout	pass	126s
TOTAL PIPELINE	pass	126s

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Commit version: c3c0876

[danistrebel]

/gcbrun

[apigee-devrel-bot]

Pipeline Report

Pipeline	Result	Elapsed Wall Time
references/js-callout	pass	112s
TOTAL PIPELINE	pass	112s

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Commit version: 469b5f0