Entity references in attribute values do not compile

``` scala
import org.scalajs.dom.document
import com.thoughtworks.binding._

@dom def foo = {
  <input type="button" value="&copy; Yang, Bo"/>
}

dom.render(document.body, foo)
```
https://scalafiddle.io/sf/xOtA9BQ/0

```
ScalaFiddle.scala:5: error: type mismatch;
 found   : .this.com.thoughtworks.binding.Binding.BindingSeq[raw.this.Text]
 required: String
```