Improve requestUri handling in ReactorUriHelper

Closes gh-36893

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

@@ -146,6 +146,9 @@ abstract class ReactorUriHelper {
 				return;
 			}
 		}
+		if (length > 0 && uri.charAt(0) != '/') {
+			builder.append('/');
+		}
 		builder.append(uri);
 	}
 

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

@@ -68,6 +68,21 @@ class ReactorUriHelperTests {
 				.hasToString("http://example.org/path");
 	}
 
+	@Test
+	void requestUriWithoutLeadingSlash() throws URISyntaxException {
+		given(nettyRequest.scheme()).willReturn("http");
+		given(nettyRequest.hostName()).willReturn("example.org");
+		given(nettyRequest.hostPort()).willReturn(80);
+		given(nettyRequest.uri()).willReturn("foo/bar");
+
+		URI uri = ReactorUriHelper.createUri(nettyRequest);
+		assertThat(uri).hasScheme("http")
+				.hasHost("example.org")
+				.hasPort(-1)
+				.hasPath("/foo/bar")
+				.hasToString("http://example.org/foo/bar");
+	}
+
 	@ParameterizedTest(name = "{displayName}({arguments})")
 	@CsvSource(delimiter='|', value = {
 			"/prefix           | /prefix/",