Add headerDoesNotExist() to MockRestRequestMatchers

Prior to this commit, one could not test for the absence of a specific
HTTP header in a request.

This commit adds a headerDoesNotExist() method in MockRestRequestMatchers.

Closes gh-23721

spring-test/src/test/java/org/springframework/test/web/client/match/MockRestRequestMatchersTests.java

@@ -19,6 +19,7 @@ package org.springframework.test.web.client.match;
 import java.net.URI;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.List;
 
 import org.junit.jupiter.api.Test;
 
@@ -93,6 +94,19 @@ public class MockRestRequestMatchersTests {
 		MockRestRequestMatchers.header("foo", "bar", "baz").match(this.request);
 	}
 
+	@Test
+	public void headerDoesNotExist() throws Exception {
+		MockRestRequestMatchers.headerDoesNotExist(null).match(this.request);
+		MockRestRequestMatchers.headerDoesNotExist("").match(this.request);
+		MockRestRequestMatchers.headerDoesNotExist("foo").match(this.request);
+
+		List<String> values = Arrays.asList("bar", "baz");
+		this.request.getHeaders().put("foo", values);
+		assertThatThrownBy(() -> MockRestRequestMatchers.headerDoesNotExist("foo").match(this.request))
+			.isInstanceOf(AssertionError.class)
+			.hasMessage("Expected header <foo> not to exist, but it exists with values: " + values);
+	}
+
 	@Test
 	public void headerMissing() throws Exception {
 		assertThatThrownBy(() -> MockRestRequestMatchers.header("foo", "bar").match(this.request))