rpc: make slightly more robust fix to authentication retries
rpc: make slightly more robust fix to authentication retries

The previous fix cleaned up from the previous auth attempt by removing a
specific key from the header. This change simply re-allocates the header.

Change-Id: I0ccd3cd92b624bd5da775dc58f02ad1517b057b4
Reviewed-on: https://upspin-review.googlesource.com/8067
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

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