Fix default value of `og:description` in `open_graph`

By default, `og:description` should be `page.excerpt` or first 200 characters of `page.content`.

However, in the current implementation, `page.excerpt` will always be overwritten by `page.content`.
Merge pull request #578 from sun-zheng-an/bug/open_graph

Fix default value of `og:description` in `open_graph`