send-pull-request: Use current date in mail headers
Some users experience problems viewing the pull requests as a sequential
mail series due to the script using the git commit date for the patches
and today's date for the cover letter.
Address this by renaming the email Date: header to Old-Date: and adding
a new Date: header with a current timestamp.
Signed-off-by: Darren Hart <dvhart@linux.intel.com> Reported-by: Gary Thomas <gary@mlbassoc.com> Cc: Josh Lock <josh@linux.intel.com>
