Added CompositeHeaderWriter

1. Added new CompositeHeaderWriter
2. Improvement in HeaderWriterFilter using CompositeHeaderWriter.

Fixes: gh-6453

web/src/test/java/org/springframework/security/web/header/writers/CompositeHeaderWriterTests.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2002-2019 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.web.header.writers;
+
+import java.util.Arrays;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import org.springframework.mock.web.MockHttpServletRequest;
+import org.springframework.mock.web.MockHttpServletResponse;
+import org.springframework.security.web.header.HeaderWriter;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for class {@link CompositeHeaderWriter}/.
+ *
+ * @author Ankur Pathak
+ * @since 5.2
+ */
+public class CompositeHeaderWriterTests {
+	private MockHttpServletRequest request;
+
+	private MockHttpServletResponse response;
+
+	private CompositeHeaderWriter writer;
+
+	@Before
+	public void setup() {
+		this.request = new MockHttpServletRequest();
+		this.response = new MockHttpServletResponse();
+		HeaderWriter writerA = (request, response) -> {
+			if (!response.containsHeader("A")) {
+				response.setHeader("A", "a");
+			}
+		};
+
+		HeaderWriter writerB = (request, response) -> {
+			if (!response.containsHeader("B")) {
+				response.setHeader("B", "b");
+			}
+		};
+		this.writer = new CompositeHeaderWriter(Arrays.asList(writerA, writerB));
+	}
+
+
+	@Test
+	public void doCompositeHeaderWrite(){
+		this.writer.writeHeaders(request, response);
+		assertThat(this.response.containsHeader("A")).isTrue();
+		assertThat(this.response.containsHeader("B")).isTrue();
+		assertThat(this.response.getHeader("A")).isEqualTo("a");
+		assertThat(this.response.getHeader("B")).isEqualTo("b");
+	}
+}