headers: reset the requests counter at transfer start

If not, reusing an easy handle to do a subsequent transfer would
continue the counter from the previous invoke, which then would mak

headers: reset the requests counter at transfer start

If not, reusing an easy handle to do a subsequent transfer would
continue the counter from the previous invoke, which then would make use
of the header API difficult/impossible as the request counter
mismatched.

Add libtest 1947 to verify.

Reported-by: Andrew Lambert
Fixes #9424
Closes #9447

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