Fix RfcUriParser parsing for single char fragments

Prior to this commit, the `RfcUriParser` would ignore URI fragments if
their length is < 2. This commit fixes the length check to allow for
single char fragments when parsing URIs.

Fixes gh-36029

spring-web/src/main/java/org/springframework/web/util/RfcUriParser.java

@@ -563,7 +563,7 @@ abstract class RfcUriParser {
 		}
 
 		public void captureFragmentIfNotEmpty() {
-			if (this.index > this.componentIndex + 1) {
+			if (this.index > this.componentIndex) {
 				this.fragment = captureComponent("fragment");
 			}
 		}

spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java

@@ -914,4 +914,13 @@ class UriComponentsBuilderTests {
 		assertThat(uri.toString()).isEqualTo("ws://localhost:7777/test");
 	}
 
+	@ParameterizedTest // gh-36029
+	@EnumSource
+	void singleCharFragment(ParserType parserType) {
+		URI uri = UriComponentsBuilder
+				.fromUriString("https://localhost/resource#a", parserType)
+				.build().toUri();
+		assertThat(uri.toString()).isEqualTo("https://localhost/resource#a");
+	}
+
 }