Fail V4 signature requests with InvalidArgument
Fail V4 signature requests with InvalidArgument

This allows V4 clients like s3cmd to retry with V2 signatures.
References #24.

kahing/s3proxy,andrewgaul/s3proxy,flandr/s3proxy,flandr/s3proxy,bouncestorage/s3proxy,timuralp/s3proxy,kahing/s3proxy,andrewgaul/s3proxy,timuralp/s3proxy,bouncestorage/s3proxy