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/object-handlers.go

@@ -445,10 +445,23 @@ func (api objectAPIHandlers) PutObjectHandler(w http.ResponseWriter, r *http.Req
 	// Extract metadata to be saved from incoming HTTP header.
 	metadata := extractMetadataFromHeader(r.Header)
 	if rAuthType == authTypeStreamingSigned {
-		// Make sure to delete the content-encoding parameter
-		// for a streaming signature which is set to value
-		// "aws-chunked"
-		delete(metadata, "content-encoding")
+		if contentEncoding, ok := metadata["content-encoding"]; ok {
+			contentEncoding = trimAwsChunkedContentEncoding(contentEncoding)
+			if contentEncoding != "" {
+				// Make sure to trim and save the content-encoding
+				// parameter for a streaming signature which is set
+				// to a custom value for example: "aws-chunked,gzip".
+				metadata["content-encoding"] = contentEncoding
+			} else {
+				// Trimmed content encoding is empty when the header
+				// value is set to "aws-chunked" only.
+
+				// Make sure to delete the content-encoding parameter
+				// for a streaming signature which is set to value
+				// for example: "aws-chunked"
+				delete(metadata, "content-encoding")
+			}
+		}
 	}
 
 	// Make sure we hex encode md5sum here.