Change X-Requested-With header to "Fetch" in FetchHttpClient

[val1984]

Change X-Requested-With header to "Fetch" in FetchHttpClient

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Change X-Requested-With header to "Fetch" from "XmlHttpRequest" in FetchHttpClient

Description

In its initial version, FetchHttpClient used to set X-Requested-With header to "Fetch" but it was changed in a later commit and it currently sets it as "XmlHttpRequest" which makes it harder to know which client is really used.

Fixes #44933

[ghost]

Thanks for your PR, @val1984. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[BrennanConroy]

Please file an issue/make sure there is an issue before making a PR so that potential changes can be discussed first.
https://github.com/dotnet/aspnetcore/blob/main/CONTRIBUTING.md#before-submitting-the-pull-request

[val1984]

Please file an issue/make sure there is an issue before making a PR so that potential changes can be discussed first. https://github.com/dotnet/aspnetcore/blob/main/CONTRIBUTING.md#before-submitting-the-pull-request

@BrennanConroy Thank you for the reply. I created issue #44933

[BrennanConroy]

Closing as the conversation in #44933 didn't convince us of the need for this change.