Avoid duplicate Accept header values in RestTemplate

Prior to this commit, the various `HttpMessageConverter` instances
configured for a given `RestTemplate` instance could all contribute
`MediaType` values to the "Accept:" request header.

This could lead to duplicate media types in that request header,
cluttering for the HTTP request for no reason.

This commit ensures that only distinct values are added to the request.

Issue: SPR-16690
Closes gh-22320
Closes gh-21231

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

 /*
- * 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.
@@ -20,6 +20,7 @@ import java.io.IOException;
 import java.lang.reflect.Type;
 import java.net.URI;
 import java.util.ArrayList;
+import java.util.LinkedHashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
@@ -767,7 +768,7 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 				if (this.responseType instanceof Class) {
 					responseClass = (Class<?>) this.responseType;
 				}
-				List<MediaType> allSupportedMediaTypes = new ArrayList<MediaType>();
+				Set<MediaType> allSupportedMediaTypes = new LinkedHashSet<MediaType>();
 				for (HttpMessageConverter<?> converter : getMessageConverters()) {
 					if (responseClass != null) {
 						if (converter.canRead(responseClass, null)) {
@@ -782,11 +783,12 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 					}
 				}
 				if (!allSupportedMediaTypes.isEmpty()) {
-					MediaType.sortBySpecificity(allSupportedMediaTypes);
+					List<MediaType> result = new ArrayList<MediaType>(allSupportedMediaTypes);
+					MediaType.sortBySpecificity(result);
 					if (logger.isDebugEnabled()) {
 						logger.debug("Setting request Accept header to " + allSupportedMediaTypes);
 					}
-					request.getHeaders().setAccept(allSupportedMediaTypes);
+					request.getHeaders().setAccept(result);
 				}
 			}
 		}

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

 /*
- * Copyright 2002-2018 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.
@@ -19,6 +19,8 @@ package org.springframework.web.client;
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.net.URI;
+import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.HashMap;
@@ -43,6 +45,8 @@ 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.http.converter.StringHttpMessageConverter;
+import org.springframework.util.StringUtils;
 import org.springframework.web.util.DefaultUriTemplateHandler;
 
 import static org.hamcrest.MatcherAssert.assertThat;
@@ -878,4 +882,24 @@ public class RestTemplateTests {
 		verify(response).close();
 	}
 
+	@Test
+	public void acceptHeaderValueShouldBeNotDuplicated() throws Exception {
+		final StringHttpMessageConverter utf8HttpMessageConverter = new StringHttpMessageConverter(StandardCharsets.UTF_8);
+		final StringHttpMessageConverter iso88591HttpMessageConverter = new StringHttpMessageConverter(StandardCharsets.ISO_8859_1);
+
+		final RestTemplate multipleEncodingTemplate = new RestTemplate(Arrays.asList(utf8HttpMessageConverter, iso88591HttpMessageConverter));
+		multipleEncodingTemplate.setRequestFactory(requestFactory);
+		given(requestFactory.createRequest(new URI("http://example.com"), GET)).willReturn(request);
+
+		final HttpHeaders requestHeaders = new HttpHeaders();
+		given(request.getHeaders()).willReturn(requestHeaders);
+		given(request.execute()).willReturn(response);
+
+		final HttpHeaders responseHeaders = new HttpHeaders();
+		given(response.getHeaders()).willReturn(responseHeaders);
+
+		multipleEncodingTemplate.getForObject("http://example.com", String.class);
+		assertEquals("text/plain, */*", StringUtils.collectionToDelimitedString(request.getHeaders().get("accept"), ","));
+	}
+
 }