Fix og:image being detected

For some reason, this code was using `property=` to match the tag, but that’s incorrect:

```
>>> text = "<html><meta name=\"og:image\" content=\"text.png\" /></html>"
>>> import BeautifulSoup
>>> soup = BeautifulSoup.BeautifulSoup(text)
>>> soup.find('meta')
<meta name="og:image" content="text.png" />
>>> soup.find('meta', property='og:image')
>>> soup.find('meta', attrs={'name':'og:image'})
<meta name="og:image" content="text.png" />
```