Always set the sigv4 date to the current time

Part of the reason this change looks more complicated than
you would expect is because we use the sigv4 test suite
(http://docs.aws.amazon.com/general/latest/gr/signature-v4-test-suite.html)
to test our implementation.  However, these tests use a Date header
whereas previous versions used an X-Amz-Date.

We can possibly investigate always using Date instead of X-Amz-Date,
but we'd need to make sure we aren't introducing any regressions.