HTTP auth "accidentally" worked before this cleanup since the code would
always overwrite the connection credentials with the credentials from
the most recent transfer and since HTTP auth is typically done first
thing, this has not been an issue. It was still wrong and subject to
possible race conditions or future breakage if the sequence of functions
would change.
The data.set.str[] strings MUST remain unmodified exactly as set by the
user, and the credentials to use internally are instead set/updated in
state.aptr.*
Added test 675 to verify different credentials used in two requests done
over a reused HTTP connection, which previously behaved wrongly.
Fixes#6542Closes#6545
