Default headers overriding existing headers *FIX*

[mP1]

• RestClient
  -- remove old Set tracking "added" headers.
  -- use HttpHeaders.containsHeader instead.
  -- Fixes Make headers case-insensitive #22623

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[javanna]

hi @mP1 thanks for your PR. We already have a PR under review that fixes the same problem, see #29419. The user is addressing comments so we would prefer to integrate the existing PR. Your proposed solution is appealing, the problem here and in the other PR is though that they are both missing tests. I am closing this and I will reopen in case that other PR doesn't go anywhere.