sign/streaming: Content-Encoding is not set in newer aws-java-sdks (#3986)

We can't use Content-Encoding to verify if `aws-chunked` is set
or not. Just use 'streaming' signature header instead.

While this is considered mandatory, on the contrary aws-sdk-java
doesn't set this value

http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-streaming.html

```
Set the value to aws-chunked.
```

We will relax it and behave appropriately. Also this PR supports
saving custom encoding after trimming off the `aws-chunked`
parameter.

Fixes #3983

cmd/auth-handler.go

@@ -64,7 +64,6 @@ func isRequestPostPolicySignatureV4(r *http.Request) bool {
 // Verify if the request has AWS Streaming Signature Version '4'. This is only valid for 'PUT' operation.
 func isRequestSignStreamingV4(r *http.Request) bool {
 	return r.Header.Get("x-amz-content-sha256") == streamingContentSHA256 &&
-		r.Header.Get("content-encoding") == streamingContentEncoding &&
 		r.Method == httpPUT
 }