lib: fix aws-sigv4 having date header twice in some cases

When the user was providing the header X-XXX-Date, the header was
re-added during signature computation, and we had it twice in

lib: fix aws-sigv4 having date header twice in some cases

When the user was providing the header X-XXX-Date, the header was
re-added during signature computation, and we had it twice in the
request.

Reported-by: apparentorder@users.noreply.github.com

Signed-off-by: Matthias Gatto <matthias.gatto@outscale.com>

Fixes: https://github.com/curl/curl/issues/11738
Closes: https://github.com/curl/curl/pull/11754

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