HttpServerResponse.close() should be idempotent

[NiteshKant]

HttpServerResponse.close() is not idempotent and hence can not be called multiple times.

The problem with this is that since HttpConnectionHandler finally calls close at the end of request processing, a request handler can not close the response. Instead what the request handlers end up doing is call flush() on the response in a code similar to this:

new HttpServerBuilder<ByteBuf, ByteBuf>(7008, new RequestHandler<ByteBuf, ByteBuf>() {
            @Override
            public Observable<Void> handle(HttpServerRequest<ByteBuf> request, HttpServerResponse<ByteBuf> response) {
                return response.writeStringAndFlush("Hello World");
            }
        }, true)

The above code will invoke two flushes per request and hence cause two underlying socket writes as opposed to ideally a single write.

If we make HttpServerResponse.close() idempotent the above code can be changed to

new HttpServerBuilder<ByteBuf, ByteBuf>(7008, new RequestHandler<ByteBuf, ByteBuf>() {
            @Override
            public Observable<Void> handle(HttpServerRequest<ByteBuf> request, HttpServerResponse<ByteBuf> response) {
                response.writeString("Hello World");
                return response.close();
            }
        }, true)

Now, HttpConnectionHandler calling close will short-circuit (since the response is already closed) and hence will reduce one flush.