Fix Map.put contract for HeadersAdapter impl.

This commit fixes the `Map.put` contract for both Reactor Netty and
Tomcat specific `HeadersAdapter` implementations.

Issue: SPR-17415

spring-web/src/main/java/org/springframework/http/server/reactive/NettyHeadersAdapter.java

@@ -124,7 +124,7 @@ class NettyHeadersAdapter implements MultiValueMap<String, String> {
 	@Override
 	public List<String> put(String key, @Nullable List<String> value) {
 		List<String> previousValues = this.headers.getAll(key);
-		this.headers.add(key, value);
+		this.headers.set(key, value);
 		return previousValues;
 	}
 

spring-web/src/main/java/org/springframework/http/server/reactive/TomcatHeadersAdapter.java

@@ -138,6 +138,7 @@ class TomcatHeadersAdapter implements MultiValueMap<String, String> {
 	@Nullable
 	public List<String> put(String key, List<String> value) {
 		List<String> previousValues = get(key);
+		this.headers.removeHeader(key);
 		value.forEach(v -> this.headers.addValue(key).setString(v));
 		return previousValues;
 	}

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

@@ -16,12 +16,14 @@
 
 package org.springframework.http.server.reactive;
 
+import java.util.Arrays;
 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.After;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.Parameterized;
@@ -57,6 +59,11 @@ public class HeadersAdaptersTests {
 		};
 	}
 
+	@After
+	public void tearDown() {
+		this.headers.clear();
+	}
+
 	@Test
 	public void getWithUnknownHeaderShouldReturnNull() {
 		assertNull(this.headers.get("Unknown"));
@@ -96,4 +103,12 @@ public class HeadersAdaptersTests {
 		assertEquals("first", this.headers.get("TestHeader").get(0));
 	}
 
-}
+	@Test
+	public void putShouldOverrideExisting() {
+		this.headers.add("TestHeader", "first");
+		this.headers.put("TestHeader", Arrays.asList("override"));
+		assertEquals("override", this.headers.getFirst("TestHeader"));
+		assertEquals(1, this.headers.get("TestHeader").size());
+	}
+
+}