Allow interceptors to add existing header values

Additional commit to provide support of requests
that have a body.

Issue: SPR-15066

spring-web/src/main/java/org/springframework/web/client/RestTemplate.java

@@ -922,6 +922,7 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 				Class<?> requestBodyClass = requestBody.getClass();
 				Type requestBodyType = (this.requestEntity instanceof RequestEntity ?
 						((RequestEntity<?>)this.requestEntity).getType() : requestBodyClass);
+				HttpHeaders httpHeaders = httpRequest.getHeaders();
 				HttpHeaders requestHeaders = this.requestEntity.getHeaders();
 				MediaType requestContentType = requestHeaders.getContentType();
 				for (HttpMessageConverter<?> messageConverter : getMessageConverters()) {
@@ -930,7 +931,9 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 								(GenericHttpMessageConverter<Object>) messageConverter;
 						if (genericConverter.canWrite(requestBodyType, requestBodyClass, requestContentType)) {
 							if (!requestHeaders.isEmpty()) {
-								httpRequest.getHeaders().putAll(requestHeaders);
+								for (Map.Entry<String, List<String>> entry : requestHeaders.entrySet()) {
+									httpHeaders.put(entry.getKey(), new LinkedList<>(entry.getValue()));
+								}
 							}
 							if (logger.isDebugEnabled()) {
 								if (requestContentType != null) {
@@ -948,7 +951,9 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 					}
 					else if (messageConverter.canWrite(requestBodyClass, requestContentType)) {
 						if (!requestHeaders.isEmpty()) {
-							httpRequest.getHeaders().putAll(requestHeaders);
+							for (Map.Entry<String, List<String>> entry : requestHeaders.entrySet()) {
+								httpHeaders.put(entry.getKey(), new LinkedList<>(entry.getValue()));
+							}
 						}
 						if (logger.isDebugEnabled()) {
 							if (requestContentType != null) {

spring-web/src/test/java/org/springframework/web/client/RestTemplateTests.java

@@ -826,7 +826,7 @@ public class RestTemplateTests {
 	}
 
 	@Test // SPR-15066
-	public void requestInterceptorCanAddExistingHeaderValue() throws Exception {
+	public void requestInterceptorCanAddExistingHeaderValueWithoutBody() throws Exception {
 		ClientHttpRequestInterceptor interceptor = (request, body, execution) -> {
 			request.getHeaders().add("MyHeader", "MyInterceptorValue");
 			return execution.execute(request, body);
@@ -851,4 +851,35 @@ public class RestTemplateTests {
 		verify(response).close();
 	}
 
+	@Test // SPR-15066
+	public void requestInterceptorCanAddExistingHeaderValueWithBody() throws Exception {
+		ClientHttpRequestInterceptor interceptor = (request, body, execution) -> {
+			request.getHeaders().add("MyHeader", "MyInterceptorValue");
+			return execution.execute(request, body);
+		};
+		template.setInterceptors(Collections.singletonList(interceptor));
+
+		MediaType contentType = MediaType.TEXT_PLAIN;
+		given(converter.canWrite(String.class, contentType)).willReturn(true);
+		given(requestFactory.createRequest(new URI("http://example.com"), HttpMethod.POST)).willReturn(request);
+		String helloWorld = "Hello World";
+		HttpHeaders requestHeaders = new HttpHeaders();
+		given(request.getHeaders()).willReturn(requestHeaders);
+		converter.write(helloWorld, contentType, request);
+		given(request.execute()).willReturn(response);
+		given(errorHandler.hasError(response)).willReturn(false);
+		HttpStatus status = HttpStatus.OK;
+		given(response.getStatusCode()).willReturn(status);
+		given(response.getStatusText()).willReturn(status.getReasonPhrase());
+
+		HttpHeaders entityHeaders = new HttpHeaders();
+		entityHeaders.setContentType(contentType);
+		entityHeaders.add("MyHeader", "MyEntityValue");
+		HttpEntity<String> entity = new HttpEntity<>(helloWorld, entityHeaders);
+		template.exchange("http://example.com", HttpMethod.POST, entity, Void.class);
+		assertThat(requestHeaders.get("MyHeader"), contains("MyEntityValue", "MyInterceptorValue"));
+
+		verify(response).close();
+	}
+
 }