describe_db_instances db_instance_identifier option does not function properly

[fmcgeough]

Option to describe_db_instances doesn't return expected values.

The following code will work:

  def describe_rds_instance(rds_db_identifier, keys) do 
    options = [{:DBInstanceIdentifier,  rds_db_identifier}]
    ExAws.RDS.describe_db_instances(options) |> ExAws.request(keys)
  end

but describe_db_instances_opts type has:

{:db_instance_identifier, binary}

If this is passed in as a parameter its ignored and every RDS instance for your requested account/region will be returned. The code currently ends up sending "DbInstanceIdentifier" and since the API is case-sensitive it doesn't match on this key.