fix: fix ic-http doc test

[maksymar]

This PR fixes ic-http doc test by explicitly using u8 type.

[github-actions]

canbench 🏋

No significant performance changes detected ✅

./canbench_results.yml is up to date ✅

~/work/bitcoin-canister/bitcoin-canister/scripts ~/work/bitcoin-canister/bitcoin-canister
~/work/bitcoin-canister/bitcoin-canister

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Benchmark: insert_300_blocks
  total:
    instructions: 561.77 M (no change)
    heap_increase: 6 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: get_metrics
  total:
    instructions: 87.00 M (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers
  total:
    instructions: 3.90 B (no change)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers_multiple_times
  total:
    instructions: 13.90 B (no change)
    heap_increase: 7 pages (no change)
    stable_memory_increase: 0 pages (no change)

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