Allow interceptors to add existing header values

Provide a fully mutable HttpHeaders to ClientHttpRequestInterceptors
of a RestTemplate when headers are set using HttpEntity. This avoids
UnsupportedOperationException if both HttpEntity and
ClientHttpRequestInterceptor add values for the same HTTP header.

Issue: SPR-15066

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

@@ -21,6 +21,7 @@ import java.lang.reflect.Type;
 import java.net.URI;
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -909,7 +910,9 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 				HttpHeaders httpHeaders = httpRequest.getHeaders();
 				HttpHeaders requestHeaders = this.requestEntity.getHeaders();
 				if (!requestHeaders.isEmpty()) {
-					httpHeaders.putAll(requestHeaders);
+					for (Map.Entry<String, List<String>> entry : requestHeaders.entrySet()) {
+						httpHeaders.put(entry.getKey(), new LinkedList<>(entry.getValue()));
+					}
 				}
 				if (httpHeaders.getContentLength() < 0) {
 					httpHeaders.setContentLength(0L);

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

@@ -39,12 +39,15 @@ import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.client.ClientHttpRequest;
 import org.springframework.http.client.ClientHttpRequestFactory;
+import org.springframework.http.client.ClientHttpRequestInterceptor;
 import org.springframework.http.client.ClientHttpResponse;
 import org.springframework.http.converter.GenericHttpMessageConverter;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.util.StreamUtils;
 import org.springframework.web.util.DefaultUriBuilderFactory;
 
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.collection.IsIterableContainingInOrder.contains;
 import static org.junit.Assert.*;
 import static org.mockito.BDDMockito.*;
 import static org.springframework.http.HttpMethod.POST;
@@ -822,4 +825,30 @@ public class RestTemplateTests {
 		verify(response).close();
 	}
 
+	@Test // SPR-15066
+	public void requestInterceptorCanAddExistingHeaderValue() throws Exception {
+		ClientHttpRequestInterceptor interceptor = (request, body, execution) -> {
+			request.getHeaders().add("MyHeader", "MyInterceptorValue");
+			return execution.execute(request, body);
+		};
+		template.setInterceptors(Collections.singletonList(interceptor));
+
+		given(requestFactory.createRequest(new URI("http://example.com"), HttpMethod.POST)).willReturn(request);
+		HttpHeaders requestHeaders = new HttpHeaders();
+		given(request.getHeaders()).willReturn(requestHeaders);
+		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.add("MyHeader", "MyEntityValue");
+		HttpEntity<Void> entity = new HttpEntity<>(null, entityHeaders);
+		template.exchange("http://example.com", HttpMethod.POST, entity, Void.class);
+		assertThat(requestHeaders.get("MyHeader"), contains("MyEntityValue", "MyInterceptorValue"));
+
+		verify(response).close();
+	}
+
 }