undefined attribute values are converted to strings

When attribute values are `undefined` or `null` they are appended as strings 'undefined' or 'null'.

For example this code:

``` javascript
var none // unassigned
hx`...`
```

Will result in html like this:

``` html

Whereas `virtual-hyperscript` will ignore the value and not add it to the class list.

I'm not sure if this is the correct behavior, but i expected it to do the same as `virtual-hyperscript`. This can be achieved by the developer by supplying a custom concat function, or by ensuring that undefined values are never used, but i think this should be handled by default?

If it is a bug, I think it can be fixed in the [concat function](https://github.com/substack/hyperx/blob/2.0.4/index.js#L13), by making undefined parameters default to empty strings (''). However maybe there is a better way to do it?