Presigned post fails when given acl value

[jeffreyplusplus]

Current behavior

ExAws.S3.presigned_post/4 errors when it is called with options that include a specific value for acl.

>> options = [acl: "public-read"]
>> ExAws.S3.presigned_post(config, "bucket", nil, options)
>> ** (ArgumentError) argument error
      :erlang.++(%{"acl" => "public-read"}, [])
      (ex_aws_s3 2.3.0) lib/ex_aws/s3/utils.ex:320: ExAws.S3.Utils.build_amz_post_policy/6
      (ex_aws_s3 2.3.0) lib/ex_aws/s3.ex:1287: ExAws.S3.presigned_post/4

Expected behavior

ExAws.S3.presigned_post/4 processes and includes acl configuration in return value