Rds download complete db logfile

Fixed to return io.ReadCloser instead of string DownloadCompleteDBLogFile function.

Usage
--------

```
  resp_body, err := rds_client.DownloadCompleteDBLogFile(
    db_instance_identifier,
    log_file_name,
  )
  if err != nil {
    log.Panic(err)
  }
  defer resp_body.Close()
  body, err := ioutil.ReadAll(resp_body)
  if err != nil {
    // some action
  }
  fmt.Print(string(body))
```