Invalid properties when passing empty string to a component

[benjaminreid]

Hello,

I have a simple component with a property of foo which defaults to empty string:

Vue.component('my-component', {
    template: '<pre>{{ $data | json }}</pre>',
    props: {
        foo: {
            type: String,
            default: ''
        }
    }
});

When using this component like so:

<my-component></my-component>

The result is as expected:

{
  "foo": ""
}

But when passing an empty string as a property value (I know this could be worked around, but it's a scenario that I've run into) like so:

<my-component foo=""></my-component>

The result is as follows with this error [Vue warn]: Invalid prop: type check failed for foo="". Expected String, got Number..

{}

I'd expect the property to still be an empty string. Is this a bug, or expected behaviour?

Fiddle for example: http://jsfiddle.net/6szLn2fm/