fix(rest): too many debug headers

[coryan]

We were injecting a :curl-peer pseudo-header (usually used for troubleshooting) for each Read() call. The existing tests did not catch this because this headers were never exposed to the application layer either.

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This change is 

[google-cloud-cpp-bot]

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For commit: 2ab72971ddc22704b7f11cd6c8eae4dd89b6acf1

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[codecov]

Codecov Report

Base: 94.22% // Head: 94.22% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d778134) compared to base (e90fd34).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10054   +/-   ##
=======================================
  Coverage   94.22%   94.22%           
=======================================
  Files        1527     1528    +1     
  Lines      141822   141886   +64     
=======================================
+ Hits       133633   133696   +63     
- Misses       8189     8190    +1     

Impacted Files	Coverage Δ
google/cloud/internal/curl_http_payload.h	100.00% <ø> (ø)
google/cloud/internal/curl_http_payload.cc	100.00% <100.00%> (ø)
google/cloud/internal/curl_impl.cc	88.18% <100.00%> (-0.03%)	⬇️
...loud/internal/curl_rest_client_integration_test.cc	98.41% <100.00%> (+0.10%)	⬆️
...torage/tests/object_read_large_integration_test.cc	100.00% <100.00%> (ø)
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
...integration_tests/schema_admin_integration_test.cc	98.88% <0.00%> (-1.12%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-0.17%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (+0.66%)	⬆️

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: d778134ac3f3b42ddd592e7dc4609e892fd516f6

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• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[scotthart]

Reviewed 6 of 7 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)