RxJs 7 setting X-Requested-With by default when RxJs 6 did not

Describe the bug

Reraise of https://github.com/ReactiveX/rxjs/issues/6663

The X-Requested-With header is still getting set in v7 with default parameters to an AJAX call where with RxJs 6 it did not, constituting a breaking change when using with a server that does not support preflight requests but does correctly return an Access-Control-Allow-Origin.

I've enhanced the reproductions to try show it more clearly in the codepen.

Expected behavior

Default parameter AJAX call to a non-same-origin endpoint which does not support Preflight but does return the correct allow headers to succeed. In other words - do not send a custom header in the request.

Reproduction code

Working:
6.6.7: https://codepen.io/Driskell/pen/yLovOxK

Not working:
7.4.0: https://codepen.io/Driskell/pen/MWvQymO
7.5.2: https://codepen.io/Driskell/pen/PoJyRaz
7.5.4: https://codepen.io/Driskell/pen/jOaGbwX
7.5.6: https://codepen.io/Driskell/pen/poVyyLo
8.0.0-alpha.4: https://codepen.io/Driskell/pen/NWMNNYV

Reproduction URL

No response

Version

7.5.2

Environment

No response

Additional context

No response

Start with the linked CodePen reproductions and compare the default AJAX request behavior between RxJS 6.6.7 and the affected 7.x versions. Trace the AJAX request defaults that produce X-Requested-With, then verify that a cross-origin request without that custom header succeeds against the described server.