fix metadata overwrite during a post request

During a post request, the object-server is ovewriting
the existing object metadata. This fix prevents the overwrite
of the system metadata while it allows for the user to
add/remove user metadata

Change-Id: Ic62cd064589b625ee425a9934be8766650622c13
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Reviewed-on: http://review.gluster.org/6254
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>