Preserve parameter order in DefaultServerRequest's ServletParametersMap

Prior to this commit, DefaultServerRequest's ServletParametersMap lost
the original parameter order when entrySet() was invoked.

To address that, this commit revises ServletParametersMap.entrySet() so
that it stores the results in a LinkedHashSet, thereby retaining the
original order.

Closes gh-36966

spring-webmvc/src/main/java/org/springframework/web/servlet/function/DefaultServerRequest.java

@@ -33,6 +33,7 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.Enumeration;
 import java.util.Iterator;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
@@ -443,7 +444,7 @@ class DefaultServerRequest implements ServerRequest {
 						List<String> value = Arrays.asList(entry.getValue());
 						return new SimpleImmutableEntry<>(entry.getKey(), value);
 					})
-					.collect(Collectors.toSet());
+					.collect(Collectors.toCollection(LinkedHashSet::new));
 		}
 
 		@Override

spring-webmvc/src/test/java/org/springframework/web/servlet/function/DefaultServerRequestTests.java

@@ -167,6 +167,20 @@ class DefaultServerRequestTests {
 		assertThat(request.param("foo")).contains("bar");
 	}
 
+	@Test  // gh-36966
+	void paramsEntrySetPreservesOrder() {
+		MockHttpServletRequest servletRequest = PathPatternsTestUtils.initRequest("GET", "/", true);
+		servletRequest.setParameter("charlie", "3");
+		servletRequest.setParameter("alpha", "1");
+		servletRequest.setParameter("bravo", "2");
+
+		DefaultServerRequest request = new DefaultServerRequest(servletRequest, this.messageConverters);
+
+		assertThat(request.params().entrySet())
+				.extracting(Map.Entry::getKey)
+				.containsExactly("charlie", "alpha", "bravo");
+	}
+
 	@Test
 	void multipartData() throws Exception {
 		MockPart formPart = new MockPart("form", "foo".getBytes(UTF_8));