Treat Kotlin nullable as non-required

Where `isOptional` is used, also check for `isNullable` i.e.
values are not considered required if they are Kotlin nullables:
- spring-messaging: named value method arguments
- spring-web: named value method arguments
- spring-webmvc: request parts

This means that Kotlin client code no longer has to explicity specify
"required=false" for Kotlin nullables -- this information is inferred
automatically by the framework.

Issue: SPR-14165

spring-core/src/test/kotlin/org/springframework/util/KotlinUtilsTests.kt

@@ -0,0 +1,60 @@
+package org.springframework.util
+
+import org.junit.Before
+import org.junit.Test
+
+import java.lang.reflect.Method
+
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.springframework.core.MethodParameter
+import org.springframework.util.KotlinUtils.*
+
+/**
+ * Unit tests for [KotlinUtils].
+
+ * @author Raman Gupta
+ */
+class KotlinUtilsTests {
+
+	private lateinit var methodNullable: Method
+	private lateinit var methodNonNullable: Method
+
+	@Before
+	@Throws(NoSuchMethodException::class)
+	fun setUp() {
+		methodNullable = javaClass.getMethod("methodNullable", String::class.java, java.lang.Long.TYPE)
+		methodNonNullable = javaClass.getMethod("methodNonNullable", String::class.java, java.lang.Long.TYPE)
+	}
+
+	@Test
+	fun `Is kotlin present`() {
+		// we'd have to change the build to test the opposite
+		assertTrue(isKotlinPresent())
+	}
+
+	@Test
+	fun `Are kotlin classes detected`() {
+		assertFalse(isKotlinClass(null))
+		assertFalse(isKotlinClass(MethodParameter::class.java))
+		assertTrue(isKotlinClass(javaClass))
+	}
+
+	@Test
+	fun `Obtains method return type nullability`() {
+		assertTrue(isNullable(-1, methodNullable, null))
+		assertFalse(isNullable(-1, methodNonNullable, null))
+	}
+
+	@Test
+	fun `Obtains method parameter nullability`() {
+		assertTrue(isNullable(0, methodNullable, null))
+		assertFalse(isNullable(1, methodNullable, null))
+	}
+
+	@Suppress("unused", "unused_parameter")
+	fun methodNullable(p1: String?, p2: Long): Int? = 42
+
+	@Suppress("unused", "unused_parameter")
+	fun methodNonNullable(p1: String?, p2: Long): Int = 42
+}