Handler StreamingHttpOutputMessage in XHR transport

Closes gh-23030

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/client/RestTemplateXhrTransport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,6 +27,7 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+import org.springframework.http.StreamingHttpOutputMessage;
 import org.springframework.http.client.ClientHttpRequest;
 import org.springframework.http.client.ClientHttpResponse;
 import org.springframework.lang.Nullable;
@@ -182,7 +183,13 @@ public class RestTemplateXhrTransport extends AbstractXhrTransport {
 		public void doWithRequest(ClientHttpRequest request) throws IOException {
 			request.getHeaders().putAll(this.headers);
 			if (this.body != null) {
-				StreamUtils.copy(this.body, SockJsFrame.CHARSET, request.getBody());
+				if (request instanceof StreamingHttpOutputMessage) {
+					((StreamingHttpOutputMessage) request).setBody(outputStream ->
+							StreamUtils.copy(this.body, SockJsFrame.CHARSET, outputStream));
+				}
+				else {
+					StreamUtils.copy(this.body, SockJsFrame.CHARSET, request.getBody());
+				}
 			}
 		}
 	}