Is there an existing issue for this?

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Describe the bug

FetchHttpClient.ts sets X-Requested-With header to XMLHttpRequest which makes it harder than needed to know which HTTP client was used by @microsoft/signalr JS library.

Expected Behavior

X-Requested-With header should be set to Fetch as it was in the initial implementation.

Steps To Reproduce

Make use of @microsoft/signalr JS library and check the headers sent by the lib when calling the negotiate route.

Exceptions (if any)

No response

.NET Version

No response

Anything else?

@microsoft/signalr: ^6.0.9