Adding the "Prefer" header as an allowed header by cors when cors is enabled

src/main/java/ca/uhn/fhir/jpa/starter/JpaRestfulServer.java

@@ -179,6 +179,7 @@ public class JpaRestfulServer extends RestfulServer {
             config.addAllowedHeader("Accept");
             config.addAllowedHeader("X-Requested-With");
             config.addAllowedHeader("Content-Type");
+            config.addAllowedHeader("Prefer");
 
             config.addAllowedOrigin(HapiProperties.getCorsAllowedOrigin());