Make BinderHeaders.BATCH_HEADERS final (fixes #3182)

Signed-off-by: wadhwaroh-lang <wadhwa.roh@gmail.com>

Resolves #3184

Author: wadhwaroh-lang

core/spring-cloud-stream/src/main/java/org/springframework/cloud/stream/binder/BinderHeaders.java

@@ -43,7 +43,7 @@ public final class BinderHeaders {
 	/**
 	 * Name of the Message header identifying structure for batch Message headers.
 	 */
-	public static String BATCH_HEADERS = PREFIX + "batchHeaders";
+	public static final String BATCH_HEADERS = PREFIX + "batchHeaders";
 
 	/**
 	 * Indicates the name of the target destination the binder should use if they