The existing code always picks a path style signature when v2 mode is
selected. Reviewing the history shows little information why it is done
that way.

A previous recent commit, 24cdd7f,
though it seems correct,  broke existing deployments that use GCS
(e.g. minio/mc#4547)

We should not sign v2 with path style when the endpoint is vhost.

This commit is tested with  MinIO/AWS/GCS when v2 is enabled.

Ask the server to send empty events in Listen Bucket API

With older MinIO versions, this would clear the whitespaces for the
scanner buffer, many whitespaces will make the scanner complains as it
needs a new line to clear some internal buffer.

With new MinIO version, ping=10 query will be considered and the server
will send empty events, instead of the whitespaces, each 10 seconds.

Make StatObject use the same values as GetObject for requests.

Fixes #1814

* Tests range requests
* Test multipart parts.

Requires minio/minio#17105 to pass on MinIO. Passes on AWS.

The omission of the missing JSON tags on the NoncurrentVersionExpiration
struct was causing the default value of NoncurrentDays to be shown even
when it was not being entered