Undo optimization from 12fe2c that can cause regression

Closes gh-31327

spring-web/src/main/java/org/springframework/http/converter/StringHttpMessageConverter.java

@@ -93,11 +93,7 @@ public class StringHttpMessageConverter extends AbstractHttpMessageConverter<Str
 	@Override
 	protected String readInternal(Class<? extends String> clazz, HttpInputMessage inputMessage) throws IOException {
 		Charset charset = getContentTypeCharset(inputMessage.getHeaders().getContentType());
-		long length = inputMessage.getHeaders().getContentLength();
-		byte[] bytes = (length >= 0 && length <= Integer.MAX_VALUE ?
-				inputMessage.getBody().readNBytes((int) length) :
-				inputMessage.getBody().readAllBytes());
-		return new String(bytes, charset);
+		return StreamUtils.copyToString(inputMessage.getBody(), charset);
 	}
 
 	@Override