Stringify all keys passed to options in `embed`

I previously submitted a patch that stringified the `:href` when
creating links thinking that other options passed were also stringified
during presentation. However, they were actually all left as symbols. To
maintain consistency and expectations with how Jsonite presents an
object as a whole, we should stringify all keys when possible.

Signed-off-by: David Celis <me@davidcel.is>