`Cookie.String` sanitizes values that can be deliberate fuzz payloads.
Stored cookies can also introduce a second value for the cookie under
test.
Build Cookie headers from raw fields and track the keys selected for
fuzzing. Use a filtered jar on an isolated client to retain session
updates without replacing those fields.
Fixes#7813Fixes#7826
Signed-off-by: Dwi Siswanto <[email protected]>