bug in passing complexe type

In Example: [Advanced attributes](https://codepen.io/codemeasandwich/pen/rgdvPX)

This works
```
Html`<a-person hi=${this.onHi} name="Beckett"/>`
```
This works
```
Html`<a-person hi=${this.onHi} x=${{a:2}} name="Beckett"/>`
```
This done NOT works !!
```
Html`<a-person x=${{a:2}} name="Beckett"/>`
```
 DOM out put -> `<a-person name="Beckett" x="[object Object]">`