MockHttpServletRequest preserves original Accept-Language header value

Issue: SPR-17566

spring-test/src/main/java/org/springframework/mock/web/MockHttpServletRequest.java

@@ -1013,12 +1013,14 @@ public class MockHttpServletRequest implements HttpServletRequest {
 			try {
 				HttpHeaders headers = new HttpHeaders();
 				headers.add(HttpHeaders.ACCEPT_LANGUAGE, value.toString());
-				setPreferredLocales(headers.getAcceptLanguageAsLocales());
+				List<Locale> locales = headers.getAcceptLanguageAsLocales();
+				this.locales.clear();
+				this.locales.addAll(locales);
 			}
 			catch (IllegalArgumentException ex) {
-				// Invalid Accept-Language format -> store plain header instead
-				doAddHeaderValue(name, value, true);
+				// Invalid Accept-Language format -> just store plain header
 			}
+			doAddHeaderValue(name, value, true);
 		}
 		else {
 			doAddHeaderValue(name, value, false);

spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java

@@ -341,6 +341,7 @@ public class MockHttpServletRequestTests {
 		List<Locale> actual = Collections.list(request.getLocales());
 		assertEquals(Arrays.asList(Locale.forLanguageTag("fr-ch"), Locale.forLanguageTag("fr"),
 				Locale.forLanguageTag("en"), Locale.forLanguageTag("de")), actual);
+		assertEquals(headerValue, request.getHeader("Accept-Language"));
 	}
 
 	@Test

spring-web/src/test/java/org/springframework/mock/web/test/MockHttpServletRequest.java

@@ -1013,12 +1013,14 @@ public class MockHttpServletRequest implements HttpServletRequest {
 			try {
 				HttpHeaders headers = new HttpHeaders();
 				headers.add(HttpHeaders.ACCEPT_LANGUAGE, value.toString());
-				setPreferredLocales(headers.getAcceptLanguageAsLocales());
+				List<Locale> locales = headers.getAcceptLanguageAsLocales();
+				this.locales.clear();
+				this.locales.addAll(locales);
 			}
 			catch (IllegalArgumentException ex) {
-				// Invalid Accept-Language format -> store plain header instead
-				doAddHeaderValue(name, value, true);
+				// Invalid Accept-Language format -> just store plain header
 			}
+			doAddHeaderValue(name, value, true);
 		}
 		else {
 			doAddHeaderValue(name, value, false);