Fix root path test and add more test cases

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

@@ -190,13 +190,13 @@ class DefaultPathSegmentContainer implements PathSegmentContainer {
 	}
 
 	static PathSegmentContainer subPath(PathSegmentContainer container, int fromIndex, int toIndex) {
-		if (fromIndex == toIndex) {
-			return EMPTY_PATH;
-		}
 		List<PathSegment> segments = container.pathSegments();
 		if (fromIndex == 0 && toIndex == segments.size()) {
 			return container;
 		}
+		if (fromIndex == toIndex) {
+			return EMPTY_PATH;
+		}
 
 		Assert.isTrue(fromIndex < toIndex, "fromIndex: " + fromIndex + " should be < toIndex " + toIndex);
 		Assert.isTrue(fromIndex >= 0 && fromIndex < segments.size(), "Invalid fromIndex: " + fromIndex);

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

@@ -130,6 +130,10 @@ public class DefaultPathSegmentContainerTests {
 		assertEquals("/b/c", PathSegmentContainer.subPath(path, 1).value());
 		assertEquals("/c", PathSegmentContainer.subPath(path, 2).value());
 
+		// root path
+		path = PathSegmentContainer.parse("/", UTF_8);
+		assertEquals("/", PathSegmentContainer.subPath(path, 0).value());
+
 		// trailing slash
 		path = PathSegmentContainer.parse("/a/b/", UTF_8);
 		assertEquals("/b/", PathSegmentContainer.subPath(path, 1).value());

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

@@ -39,6 +39,9 @@ public class DefaultRequestPathTests {
 		// context path only
 		testRequestPath("/a/b", "/a/b", "", false, true, false);
 
+		// root path
+		testRequestPath("/", "", "/", false, true, false);
+
 		// empty path
 		testRequestPath("", "", "", true, false, false);
 		testRequestPath("", "/", "", true, false, false);