Support noarg callable references in Kotlin beans DSL

Closes gh-23395

spring-context/src/test/kotlin/org/springframework/context/support/BeanDefinitionDslTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -156,6 +156,7 @@ class BeanDefinitionDslTests {
 		val beans = beans {
 			bean<Bar>()
 			bean(::baz)
+			bean(::foo)
 		}
 		val context = GenericApplicationContext().apply {
 			beans.initialize(this)
@@ -205,3 +206,4 @@ class FooFoo(val name: String)
 class BarBar(val foos: Collection<Foo>)
 
 fun baz(bar: Bar) = Baz(bar)
+fun foo() = Foo()