Only canonicalize header key once in each iteration

[ziyi-yan]

References to other Issues or PRs

No related issue exists yet.

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

Move canonicalizing header key to outer loop for doing it only once for each key

Other comments

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[johanbrandhorst]

Nice!

[johanbrandhorst]

Thanks for your contribution! Could you please cherry pick this against the v2 branch?

[ziyi-yan]

@johanbrandhorst No problem! I'll do it later.