http2 - compat "socket"

The compat socket getter should probably have a bit more null checks in order to be compat with old code:

i.e.

```js
  get socket() {
    return this.stream && this.stream.session && this.stream.session.socket || undefined;
  }
```

instead of 

```js
  get socket() {
    return this.stream.session.socket;
  }
```

@apapirovski 