Handle null values in MockHttpServletResponse#setHeader

Prior to this commit, `MockHttpServletResponse#setHeader` would not
remove the header entry when given a `null` value, as documented in the
Servlet API.
This commit ensures that this behavior is enforced.

Fixes gh-34466

spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java

@@ -671,7 +671,12 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public void setHeader(String name, @Nullable String value) {
-		setHeaderValue(name, value);
+		if (value == null) {
+			this.headers.remove(name);
+		}
+		else {
+			setHeaderValue(name, value);
+		}
 	}
 
 	@Override