FIX: If the `firstBlock` is a string, don't throw an exception

Sorry I don't have a test for this, because it's specific to something that Discourse adds in via our custom formatting. 

Essentially the issue is `firstBlock` can be a `String` sometimes, and in that case we don't want to shift it, we just want to append it. All grunt tests pass and this fixes the issue in Discourse.

Thanks!