Add missing change related to last commit

See gh-25821

Author: rstoyanchev

spring-messaging/src/test/java/org/springframework/messaging/support/NativeMessageHeaderAccessorTests.java

@@ -30,7 +30,11 @@
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNotSame;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
 
 /**
  * Test fixture for {@link NativeMessageHeaderAccessor}.
@@ -230,7 +234,7 @@ public void setImmutableIdempotent() {
 	}
 
 	@Test // gh-25821
-	void copyImmutableToMutable() {
+	public void copyImmutableToMutable() {
 		NativeMessageHeaderAccessor source = new NativeMessageHeaderAccessor();
 		source.addNativeHeader("foo", "bar");
 		Message<String> message = MessageBuilder.createMessage("payload", source.getMessageHeaders());
@@ -241,9 +245,9 @@ void copyImmutableToMutable() {
 		message = MessageBuilder.createMessage(message.getPayload(), target.getMessageHeaders());
 
 		MessageHeaderAccessor accessor = MessageHeaderAccessor.getMutableAccessor(message);
-		assertThat(accessor.isMutable());
+		assertTrue(accessor.isMutable());
 		((NativeMessageHeaderAccessor) accessor).addNativeHeader("foo", "baz");
-		assertThat(((NativeMessageHeaderAccessor) accessor).getNativeHeader("foo")).containsExactly("bar", "baz");
+		assertEquals(Arrays.asList("bar", "baz"), ((NativeMessageHeaderAccessor) accessor).getNativeHeader("foo"));
 	}
 
 }