Correctly handle global :expand => true in asset_tag

We just had a problem where when setting :expand to true in the configuration block would yield from (for example) `<%== stylesheet_tag 'foo'%>`:

```html
<link rel="stylesheet" href="["/assets/bar-14fb62decc150c8cce8a65e66490ffc4.css?body=1", "/assets/foo-39e72dfc94a56d798e7a13dfb2e9598c.css?body=1"]">
```

This change fixes things such that the expected

```html
<link rel="stylesheet" href="/assets/coderay-14fb62decc150c8cce8a65e66490ffc4.css?body=1">
<link rel="stylesheet" href="/assets/apidocs-39e72dfc94a56d798e7a13dfb2e9598c.css?body=1">
```

gets emitted.