Improve CsrfBeanDefinitionParser xml parsing

1. CsrfBeanDefinitionParser registers requestDataValueProcessor
if not already registered
2. Created Tests in CsrfBeanDefinitionParserTests

Fixes: gh-6423

config/src/test/java/org/springframework/security/config/http/CsrfBeanDefinitionParserTests.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2002-2018 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
+ *
+ *      http://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.security.config.http;
+
+import org.junit.Test;
+
+import org.springframework.context.support.ClassPathXmlApplicationContext;
+
+/**
+ * @author Ankur Pathak
+ */
+public class CsrfBeanDefinitionParserTests {
+	private static final String CONFIG_LOCATION_PREFIX =
+			"classpath:org/springframework/security/config/http/CsrfBeanDefinitionParserTests";
+
+	@Test
+	public void registerDataValueProcessorOnlyIfNotRegistered() throws Exception {
+		try (ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext()) {
+			context.setAllowBeanDefinitionOverriding(false);
+			context.setConfigLocation(this.xml("RegisterDataValueProcessorOnyIfNotRegistered"));
+			context.refresh();
+		}
+	}
+
+	private String xml(String configName) {
+		return CONFIG_LOCATION_PREFIX + "-" + configName + ".xml";
+	}
+}