Fix HeadersAdapters implementations

This commit harmonizes the `HeadersAdapter` implementations across all
supported servers with regards to the `get(Object key)` contract; some
server implementations are not sticking to a `Map`-like contract and
return empty `List` instead of `null` when a header is not present.

This also fixes the `size()` implementations to reflect the number of
header keys, as some implementations consider multiple values for the
same header as different entries.

Issue: SPR-17396

spring-web/src/test/java/org/springframework/http/server/reactive/HeadersAdaptersTests.java

@@ -0,0 +1,99 @@
+/*
+ * 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.http.server.reactive;
+
+import java.util.Locale;
+
+import io.netty.handler.codec.http.DefaultHttpHeaders;
+import io.undertow.util.HeaderMap;
+import org.apache.tomcat.util.http.MimeHeaders;
+import org.eclipse.jetty.http.HttpFields;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+
+import org.springframework.util.CollectionUtils;
+import org.springframework.util.LinkedCaseInsensitiveMap;
+import org.springframework.util.MultiValueMap;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
+
+/**
+ * Unit tests for {@code HeadersAdapters} {@code MultiValueMap} implementations.
+ *
+ * @author Brian Clozel
+ */
+@RunWith(Parameterized.class)
+public class HeadersAdaptersTests {
+
+	@Parameterized.Parameter(0)
+	public MultiValueMap<String, String> headers;
+
+	@Parameterized.Parameters(name = "headers [{0}]")
+	public static Object[][] arguments() {
+		return new Object[][] {
+				{CollectionUtils.toMultiValueMap(
+						new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH))},
+				{new NettyHeadersAdapter(new DefaultHttpHeaders())},
+				{new TomcatHeadersAdapter(new MimeHeaders())},
+				{new UndertowHeadersAdapter(new HeaderMap())},
+				{new JettyHeadersAdapter(new HttpFields())}
+		};
+	}
+
+	@Test
+	public void getWithUnknownHeaderShouldReturnNull() {
+		assertNull(this.headers.get("Unknown"));
+	}
+
+	@Test
+	public void getFirstWithUnknownHeaderShouldReturnNull() {
+		assertNull(this.headers.getFirst("Unknown"));
+	}
+
+	@Test
+	public void sizeWithMultipleValuesForHeaderShouldCountHeaders() {
+		this.headers.add("TestHeader", "first");
+		this.headers.add("TestHeader", "second");
+		assertEquals(1, this.headers.size());
+	}
+
+	@Test
+	public void keySetShouldNotDuplicateHeaderNames() {
+		this.headers.add("TestHeader", "first");
+		this.headers.add("OtherHeader", "test");
+		this.headers.add("TestHeader", "second");
+		assertEquals(2, this.headers.keySet().size());
+	}
+
+	@Test
+	public void containsKeyShouldBeCaseInsensitive() {
+		this.headers.add("TestHeader", "first");
+		assertTrue(this.headers.containsKey("testheader"));
+	}
+
+	@Test
+	public void addShouldKeepOrdering() {
+		this.headers.add("TestHeader", "first");
+		this.headers.add("TestHeader", "second");
+		assertEquals("first", this.headers.getFirst("TestHeader"));
+		assertEquals("first", this.headers.get("TestHeader").get(0));
+	}
+
+}