Set prefix, delimiter params even when empty

[harshavardhana]

We have never set values which are empty on the request
because they are perhaps not useful in the List query,
but this assumption is wrong when there are restricted
policies for a given user, because empty is actually
a valid value in IAM or Bucket policy conditions.

For example following condition would never work with our
ListObjects call and AWS cli would work fine.

        "Condition": {
            "StringEquals": {
                "s3:prefix": [
                    "",
                    "data/",
                    "data"
                ],
                "s3:delimiter": [
                    "/",
                    ""
                ]
            }
        }

The reason is empty or not prefix and delimiter should be
added to the query param in List operation, such that server
can use the value to validate the policies for the incoming
request.

Refer minio/minio-go#1064

[krisis]

@harshavardhana could you check why travis-CI is failing?

[harshavardhana]

@harshavardhana could you check why travis-CI is failing?

PR is ready for review @krisis @sinhaashish

[sinhaashish]

LGTM