Add a properties setter to ProblemDetail

Mainly to allow Kotlin idiomatic properties assignment.

Closes gh-31430

spring-web/src/test/kotlin/org/springframework/http/KotlinProblemDetailTests.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2002-2023 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.http
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+
+/**
+ * Kotlin tests for [ProblemDetail].
+ */
+class KotlinProblemDetailTests {
+
+	@Test // gh-31430
+	fun propertiesSetter() {
+		val problemDetail = ProblemDetail()
+		val map = mapOf("foo" to "bar")
+		problemDetail.properties = map
+		assertThat(problemDetail.properties).isEqualTo(map)
+	}
+
+}