Fix releasing bug in StringDecoder

This commit fixes an issue in StringDecoder, where, if the buffer did
not contain any delimiters, it was released before it was relayed to
any subscribers.

Closes gh-29119

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java

@@ -1258,6 +1258,26 @@ class WebClientIntegrationTests {
 				.verify();
 	}
 
+	@ParameterizedWebClientTest
+	void retrieveTextDecodedToFlux(ClientHttpConnector connector) {
+		startServer(connector);
+
+		prepareResponse(response -> response
+				.setHeader("Content-Type", "text/plain")
+				.setBody("Hey now"));
+
+		Flux<String> result = this.webClient.get()
+				.uri("/")
+				.accept(MediaType.TEXT_PLAIN)
+				.retrieve()
+				.bodyToFlux(String.class);
+
+		StepVerifier.create(result)
+				.expectNext("Hey now")
+				.expectComplete()
+				.verify(Duration.ofSeconds(3));
+	}
+
 	private <T> Mono<T> doMalformedChunkedResponseTest(
 			ClientHttpConnector connector, Function<ResponseSpec, Mono<T>> handler) {