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/test-utils_test.go

@@ -828,6 +828,25 @@ func newTestStreamingSignedBadChunkDateRequest(method, urlStr string, contentLen
 	return req, err
 }
 
+func newTestStreamingSignedCustomEncodingRequest(method, urlStr string, contentLength, chunkSize int64, body io.ReadSeeker, accessKey, secretKey, contentEncoding string) (*http.Request, error) {
+	req, err := newTestStreamingRequest(method, urlStr, contentLength, chunkSize, body)
+	if err != nil {
+		return nil, err
+	}
+
+	// Set custom encoding.
+	req.Header.Set("content-encoding", contentEncoding)
+
+	currTime := UTCNow()
+	signature, err := signStreamingRequest(req, accessKey, secretKey, currTime)
+	if err != nil {
+		return nil, err
+	}
+
+	req, err = assembleStreamingChunks(req, body, chunkSize, secretKey, signature, currTime)
+	return req, err
+}
+
 // Returns new HTTP request object signed with streaming signature v4.
 func newTestStreamingSignedRequest(method, urlStr string, contentLength, chunkSize int64, body io.ReadSeeker, accessKey, secretKey string) (*http.Request, error) {
 	req, err := newTestStreamingRequest(method, urlStr, contentLength, chunkSize, body)