Avoid duplicate flushes in String Http converter

Prior to this commit, the `StringHttpMessageConverter` would perform a
flush after writing the body, via `StreamUtils#copy`. This operation is
not needed as a flush is already performed by the abstract class.

Flush calls by HTTP message converters is being reconsidered altogether
in gh-35427, but we should first remove this extra operation.

Closes gh-36065

Author: bclozel

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

@@ -29,7 +29,6 @@
 import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.MediaType;
 import org.springframework.util.Assert;
-import org.springframework.util.StreamUtils;
 
 /**
  * Implementation of {@link HttpMessageConverter} that can read and write strings.
@@ -125,7 +124,7 @@ protected void writeInternal(String str, HttpOutputMessage outputMessage) throws
 			headers.setAcceptCharset(getAcceptedCharsets());
 		}
 		Charset charset = getContentTypeCharset(headers.getContentType());
-		StreamUtils.copy(str, charset, outputMessage.getBody());
+		outputMessage.getBody().write(str.getBytes(charset));
 	}