ListObjects does not parse metadata

[max0ne]

Hi, I noticed 2 issues using metadata:

1. ListObjects does not properly parses metadata from XML response. When I set WithMetadata to true, I'd expect all UserMetadata I previously returns. What I see is it returns metadata as a map of empty string

map[:]

2. StatObject is a workaround I found to get object metadata, however it also doesn't work perfectly. It parses metadata from response header, which capitalizes keys:

map[Foo:bar One:uno dos]

[max0ne]

This gist reproduces it
https://gist.github.com/max0ne/3bd6471f320c7d6cbaa007ba201196bd

I'm using minio-go v7.0.50

[harshavardhana]

it is fixed in #1795