Mutating element attributes leaves behind empty ones

[ghost]

I have links that I'm writing out where if they're disabled I'm leaving the href as undefined:

    render: function() {
        var href = this.props.disabled ? undefined : this.props.url;
        return <a href={href}>{this.props.title}</a>;
    }

When this first renders (disabled) the HTML DOM I get looks like this:

<a>Foo</a>

When the state changes and it's re-rendered enabled it looks like this:

<a href="#/hello">Foo</a>

All good so far, however if it then disables again, the href attribute is left in with no value:

<a href >Foo</a>

This means it still gets the styles for being a link and will receive focus.

For me it should be consistent for both disabled states.

[sophiebits]

(Closing in favor of #1448.)