Fix s3 create multipart upload for sigv4

Fixes https://github.com/boto/boto/issues/2609

The changes I made should only affect POST requests for s3 while using sigv4.  Behavior for other services will remain the same. I originally removed::

        if http_request.method == 'POST':
            return ""

in the ``canonical_query_string`` method. However, this cause integration test failures in services like ``sqs``. So, I reverted and overwrote this method specifically for s3.  CreateMultipartUpload now works for s3-sigv4.

cc @danielgtaylor @jamesls 
Yeah, I can do a better job of explaining it. We could move it to the S3-specific Sigv4 handler but it would be a lot of code copying. So we would need to further abstract it.