Some event handler converts to boolean

Hi,

```
const h = (tag, props, ...children) => ({ tag, props, children });
const html = htm.bind(h);
const dom = html`

In `props`, the `onCompositionStart` becomes `true`. The expected behavior is that the value of `onCompositionStart` and `onClick` in `props` should both be Function

Start by running the provided tagged-template reproduction and trace how event attributes are parsed into props. Done means both onCompositionStart and onClick remain Function values rather than being converted to booleans.

Written by the indexing model from the issue text.