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/main/java/org/springframework/test/web/client/match/MockRestRequestMatchers.java

@@ -185,6 +185,20 @@ public abstract class MockRestRequestMatchers {
 		};
 	}
 
+	/**
+	 * Assert that the given request header does not exist.
+	 * @since 5.2
+	 */
+	public static RequestMatcher headerDoesNotExist(String name) {
+		return request -> {
+			List<String> headerValues = request.getHeaders().get(name);
+			if (headerValues != null) {
+				fail("Expected header <" + name + "> not to exist, but it exists with values: " +
+						headerValues);
+			}
+		};
+	}
+
 	/**
 	 * Access to request body matchers.
 	 */