Workaround V4 Signing problem

403 error occurs if the space is included in the message, etc.

```go:
        sns_client := sns.New(auth, aws.APNortheast)
        pubOpt := &sns.PublishOpt{
                Message:  "aa      a  hoge",
                Subject:  "SNS_Message",
                TopicArn: "xxxxxxxxxxxxxxxxxxxxxxxxx",
        }
        resp, err := sns_client.Publish(pubOpt)
        log.Printf("resp: %#v\n\n", resp)
        log.Printf("err: %#v\n\n", err)
```

Execution result of the above

```
2014/10/04 05:35:53 resp: &sns.PublishResp{MessageId:"", ResponseMetadata:sns.ResponseMetadata{RequestId:"", BoxUsage:0}}

2014/10/04 05:35:53 err: &sns.Error{StatusCode:403, Code:"", Message:"403 Forbidden", RequestId:"e2fe9215-bf24-508b-9c76-debcb490bc6a"}
```

The cause is believed to be a matter of "Signature Version 4 Signing"

http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-query-string-auth.html
> The space character is a reserved character and must be encoded as "%20" (and not as "+").

but escape of net / url is
https://code.google.com/p/go/source/browse/src/pkg/net/url/url.go?name=go1.3beta1#198
>                case c == ' ' && mode == encodeQueryComponent:
>                        t[j] = '+'

I tried the implementation of replace "%20", 
but the 403 error still occurred. 
About this, 
Because there is a possibility of the problem of AWS side of a combination of sns and V4 signing,
its problem inquiry to amazon is ongoing.

As a workaround
I was back port "V2 signing".