Do we need a `tbody`-existence test?

I set up this branch to examine whether it is required that there be a test that checks whether a `tbody` element is added in the editor and in the parsed output, `.xhtml()`.

I've come to the conclusion that since all our tests with tables expect the existence of a `tbody` element, if ever this was not supplied, it would fail dozens of tests.

Another reason that this is not required is that the `tbody` is added in the DOM of the browser automatically, even if not supplied. I guess that this is how all browsers behave.

I did find a few tests that were missing the `tbody` element in their HTML strings. So this is worth a merge.