Attributes with a hyphen do not reflect to LitElement property

`<a-thing with-property="Foo"></a-thing>` with a property `withProperty` within `class AThing extends LitElement` does not reflect in the `withProperty` property.

Expected:
`withProperty` has value `"Foo"`

Actual:
`withProperty` is `null`

Glitch: [Code](https://glitch.com/edit/#!/hyphen-attributes?path=views/index.html:2:45), [Demo](https://hyphen-attributes.glitch.me/)

