Compatibility with RxJava 3.0

Closes gh-23208

spring-core/src/test/java/org/springframework/core/ReactiveAdapterRegistryTests.java

@@ -186,7 +186,7 @@ public class ReactiveAdapterRegistryTests {
 		Object target = getAdapter(io.reactivex.Completable.class).fromPublisher(source);
 		boolean condition = target instanceof io.reactivex.Completable;
 		assertThat(condition).isTrue();
-		assertThat(((io.reactivex.Completable) target).blockingGet()).isNull();
+		((io.reactivex.Completable) target).blockingAwait();
 	}
 
 	@Test

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingMessageConversionIntegrationTests.java

@@ -649,9 +649,8 @@ public class RequestMappingMessageConversionIntegrationTests extends AbstractReq
 		}
 
 		@PostMapping("/rxjava2-single")
-		@SuppressWarnings("deprecation")
 		public io.reactivex.Completable createWithRxJava2Single(@RequestBody io.reactivex.Single<Person> single) {
-			return single.map(persons::add).toCompletable();
+			return single.map(persons::add).ignoreElement();
 		}
 
 		@PostMapping("/flux")
@@ -665,17 +664,15 @@ public class RequestMappingMessageConversionIntegrationTests extends AbstractReq
 		}
 
 		@PostMapping("/rxjava2-observable")
-		@SuppressWarnings("deprecation")
 		public io.reactivex.Completable createWithRxJava2Observable(
 				@RequestBody io.reactivex.Observable<Person> observable) {
 
-			return observable.toList().doOnSuccess(persons::addAll).toCompletable();
+			return observable.toList().doOnSuccess(persons::addAll).ignoreElement();
 		}
 
 		@PostMapping("/flowable")
-		@SuppressWarnings("deprecation")
 		public io.reactivex.Completable createWithFlowable(@RequestBody Flowable<Person> flowable) {
-			return flowable.toList().doOnSuccess(persons::addAll).toCompletable();
+			return flowable.toList().doOnSuccess(persons::addAll).ignoreElement();
 		}
 	}