SNICallback regression.

in v0.8 the following used to work

```js
var server = https.createServer({
    SNICallback: function(hostname) {
        return certs[hostname] || certs["www.colingo.com"]
    }
}, function (req, res) { ... })
```

Doing this is probably unwise (I don't actually understand why?)

We maybe should not throw an error here for back compat?