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 
Can we document this kwarg in the method signature? It's not super obvious how this is used or why it's needed since it's in the standard SigV4 handler instead of in the S3-specific one. Also, does it make sense to move this logic directly into the S3-specific SigV4 handler? It seems to be the only piece of code that actually adds the `unmangled_req` keyword argument.