fix: Run LookupServletContainerInitializer logic in Spring Boot case (#…

…726)

(cherry picked from commit 3a5b768)

Co-authored-by: Denis <denis@vaadin.com>

vaadin-spring/src/main/java/com/vaadin/flow/spring/VaadinServletContextInitializer.java

@@ -236,7 +236,13 @@ public void failFastContextInitialized(ServletContextEvent event)
             Set<Class<?>> classes = Stream.concat(
                     findByAnnotationOrSuperType(getLookupPackages(), appContext,
                             Collections.emptyList(), getServiceTypes()),
-                    Stream.of(SpringLookupInitializer.class))
+                    // LookupInitializer is necessary here: it allows
+                    // identify Spring boot as a regular Web container (and run
+                    // LookupServletContainerInitializer logic) even though
+                    // LookupInitializer will be ignored because there
+                    // is its subclass SpringLookupInitializer provided
+                    Stream.of(LookupInitializer.class,
+                            SpringLookupInitializer.class))
                     .collect(Collectors.toSet());
             process(classes, event.getServletContext());
         }