For streaming signature do not save content-encoding in PutObject() (#3776)

Content-Encoding is set to "aws-chunked" which is an S3 specific
API value which is no meaning for an object. This is how S3
behaves as well for a streaming signature uploaded object.

cmd/auth-handler_test.go

@@ -43,6 +43,7 @@ func TestGetRequestAuthType(t *testing.T) {
 				Header: http.Header{
 					"Authorization":        []string{"AWS4-HMAC-SHA256 <cred_string>"},
 					"X-Amz-Content-Sha256": []string{streamingContentSHA256},
+					"Content-Encoding":     []string{streamingContentEncoding},
 				},
 				Method: "PUT",
 			},