Create Server without initial context

With `https`, I can create a tls server without any initial context

``` javascript
var server = https.createServer();
```

And add dynamic SNI contexts later:

``` javascript
server.addContext(hostname, { key: keyFileData, cert: certFileData });
```

When I try to do the same with http2, i get an exception.

```
options.pfx or options.key and options.cert are required!
```
