defaultProps treated differently than in preact

[ftes]

This library behaves differently than preact when dealing with undefined prop values.

Expected behaviour:
preact-render-to-string should behave like preact:

1. A default value is set for a prop (MyComponent.defaultProps.someProp = 'X')
2. undefined is passed as prop value (<MyComponent someProp={undefined}/>)
3. I expect to get the default value within the instantiated component (console.log(this.props.someProp) >> 'X')

Actual behaviour:
If undefined is passed as prop value, then this.props.someProp resolves to undefined.

Here you can see how the implementation in preact and preact-render-to-string differ:

• https://github.com/developit/preact/blob/master/src/vdom/index.js#L47

for (let i in defaultProps) {
  if (props[i]===undefined) {
    props[i] = defaultProps[i];
  }
}

• https://github.com/developit/preact-render-to-string/blob/master/src/util.js#L74

let defaultProps = vnode.nodeName.defaultProps,
  props = assign({}, defaultProps || vnode.attributes);
if (defaultProps) assign(props, vnode.attributes);

[developit]

Hi @ftes! You're right, we're not handling undefined/null correctly in render-to-string.
Would you be interested in submitting a PR? :)

[ftes]

Sure thing, PR created @developit

[developit]

Thanks, it's out in 3.8.1!