fix type of comments and text so they are Comment and Text instead of str

These two commits include (a) a very minor fix for the issue about comments/text being str instead of their special classes and (b) add additional tests.

The bugs in (a) makes it impossible to take advantage of the deleted attribute for both. Ideally, we should also include examples of deleted revisions into the test data so that we can identify these attributes in the tests. Because I didn't have examples of this offhand, I didn't add examples of deleted comments/reviseions to the test data.