Remove unnecessary conversions

Motivation:

In a `HttpConversionUtil#toHttp2Headers` a status code conversion can be replaced with using `HttpResponseStatus#codeAsText` method.

Modifications:

Apply `HttpResponseStatus#codeAsText` method.

Result:

Less allocations.

codec-http2/src/main/java/io/netty/handler/codec/http2/HttpConversionUtil.java

@@ -361,7 +361,7 @@ public final class HttpConversionUtil {
             }
         } else if (in instanceof HttpResponse) {
             HttpResponse response = (HttpResponse) in;
-            out.status(new AsciiString(Integer.toString(response.status().code())));
+            out.status(response.status().codeAsText());
         }
 
         // Add the HTTP headers which have not been consumed above