Support @Update annotation on unit-returning suspend functions

[breader124]

When I try to annotate the unit-returning suspend function with @Update annotation

@Repository
interface PersonRepository : CoroutineCrudRepository<Person, String> {
    @Update("{ \$inc: { age: 1 } }")
    suspend fun findAndIncrementAgeByName(name: String)
}

I get an exception

Caused by: java.lang.IllegalStateException: Update method may be void or return a numeric value (the number of updated documents). Offending Method: PersonRepository.findAndIncrementAgeByName
	at org.springframework.data.mongodb.repository.query.MongoQueryMethod.verify(MongoQueryMethod.java:497) ~[spring-data-mongodb-4.3.3.jar:4.3.3]
	at org.springframework.data.mongodb.repository.query.ReactiveMongoQueryMethod.verify(ReactiveMongoQueryMethod.java:164) ~[spring-data-mongodb-4.3.3.jar:4.3.3]
	at org.springframework.data.mongodb.repository.support.ReactiveMongoRepositoryFactory$MongoQueryLookupStrategy.resolveQuery(ReactiveMongoRepositoryFactory.java:184) ~[spring-data-mongodb-4.3.3.jar:4.3.3]
	at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.lookupQuery(QueryExecutorMethodInterceptor.java:115) ~[spring-data-commons-3.3.3.jar:3.3.3]
	... 42 common frames omitted

It seems to be related to the behavior of Kotlin's compiler. After the code compilation suspending functions return java.lang.Object and thus the function is not treated as a void-returning one on the Spring Data MongoDB level. When I pause the program execution with the debugger, it shows that the resultType is class kotlin.Unit. When I delete the suspend keyword from the findAndIncrementAgeByName method, it just shows void as a resultType.

I've prepared a minimal example to make it easy for you to reproduce this issue quickly. You can find the project here: https://github.com/breader124/spring-data-mongodb-issues-kotlin/tree/suspend-fun-annotated-with-update

[mp911de]

The validation fails because it uses a method that doesn't consider Unit being a surrogate for Void. Switching to Spring Data's ReflectionUtils.isVoid(…) should fix the issue.