fix: append server-timing header value if key exist when using setHeaders to avoid exception

[tolu]

Closes #14681

sorry for not waiting on a discussion or go ahead in the issue I created, but this felt like a bug and such a straight forward fix that I hope it was ok to just create the PR 🙏

Problem

setHeaders throws when attempting to add a second server-timing since the key already exist in the internal Map headers. server-timing should use append rather than set since many values are allowed.

Fix

Appends value of header server-timing if a key already exists in headers when using setHeaders. The value is concatenated in the same way that calling Headers: append() does: with a comma and a space (, ).

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