support pagination with 1st page LRO

[weidongxu-microsoft]

fix #638

generated code

    @ServiceMethod(returns = ReturnType.SINGLE)
    public Mono<PagedResponse<SiteInner>> changeVnetSinglePageAsync(
        String resourceGroupName, String name, VirtualNetworkProfile vnetInfo, Context context) {
        if (this.client.getEndpoint() == null) {
            return Mono
                .error(
                    new IllegalArgumentException(
                        "Parameter this.client.getEndpoint() is required and cannot be null."));
        }
        if (resourceGroupName == null) {
            return Mono
                .error(new IllegalArgumentException("Parameter resourceGroupName is required and cannot be null."));
        }
        if (name == null) {
            return Mono.error(new IllegalArgumentException("Parameter name is required and cannot be null."));
        }
        if (this.client.getSubscriptionId() == null) {
            return Mono
                .error(
                    new IllegalArgumentException(
                        "Parameter this.client.getSubscriptionId() is required and cannot be null."));
        }
        if (vnetInfo == null) {
            return Mono.error(new IllegalArgumentException("Parameter vnetInfo is required and cannot be null."));
        } else {
            vnetInfo.validate();
        }
        context = this.client.mergeContext(context);
        Mono<Response<Flux<ByteBuffer>>> mono =
            service
                .changeVnet(
                    this.client.getEndpoint(),
                    resourceGroupName,
                    name,
                    this.client.getSubscriptionId(),
                    this.client.getApiVersion(),
                    vnetInfo,
                    context)
                .cache();
        return Mono
            .zip(
                mono,
                this
                    .client
                    .<WebAppCollectionInner, WebAppCollectionInner>getLroResultAsync(
                        mono, this.client.getHttpPipeline(), WebAppCollectionInner.class, WebAppCollectionInner.class)
                    .last()
                    .flatMap(this.client::getLroFinalResultOrError))
            .map(
                res ->
                    new PagedResponseBase<>(
                        res.getT1().getRequest(),
                        res.getT1().getStatusCode(),
                        res.getT1().getHeaders(),
                        res.getT2().value(),
                        res.getT2().nextLink(),
                        null));
    }

[weidongxu-microsoft]

unrelated bug fix