ASSETS-38305 fix accessing header for registration conflict

[pheenomenon]

Description

For https://jira.corp.adobe.com/browse/ASSETS-38305
While debugging the 409 conflict issue, noticed that the Header in response object was read incorrectly.
Its not a Header javascript object, like the code above was assuming it to be, but a Header class.
SLACK Thread: https://cq-dev.slack.com/archives/C6841FMPU/p1714588065493699

{
  [Symbol(map)]: [Object: null prototype] {
    server: [ 'openresty' ],
    date: [ 'Thu, 02 May 2024 22:56:58 GMT' ],
    'content-type': [ 'application/json;charset=UTF-8' ],
    'content-length': [ '315' ],
    connection: [ 'close' ],
    'x-request-id': [ 'WN8dcCpL5afjudTSYZ0twjOuIOVD4g2J' ],
    'x-conflicting-id': [ '886068c3-fed5-43a3-867a-d6aaf6979fa5' ],
    'access-control-allow-origin': [ '*' ],
    'access-control-allow-methods': [ 'GET, POST, PUT, DELETE, OPTIONS, PATCH' ],
    'access-control-allow-headers': [
      'X-Requested-With, Origin, Content-Type, Accept, Accept-Language, Authorization, If-Modified-Since, X-Api-Key, X-Ims-Org-Id, x-recaptcha-challenge'
    ],
    'access-control-allow-credentials': [ 'true' ]
  }

Fixes # (issue)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.24%. Comparing base (4b06225) to head (44943f6).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/events.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #36   +/-   ##
=======================================
  Coverage   63.24%   63.24%           
=======================================
  Files           4        4           
  Lines         185      185           
=======================================
  Hits          117      117           
  Misses         68       68           

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