has_attr always returns TRUE?

``` r
> library(xml2)
> x <- xml("<foo id='one'> <bar> text <baz/> </bar> </foo>")
> x
{xml_document}
<foo>
[1] <bar> text <baz/> </bar>
> 
> xml_has_attr(x, "id")
[1] TRUE
> xml_attr(x, "id")
[1] "one"
> 
> # The following seems wrong:
> xml_has_attr(x, "dog")
[1] TRUE
> xml_attr(x, "dog")
[1] ""
```
