Turning on :tables by default for RedcarpetCompat.

@lsegal This test covers the case of enabling tables in Redcarpet by default.

I should note that in Redcarpet 1.17, [`:tables` is on by default][compat], so the test initially passed without any changes.

I bumped the Redcarpet version in the Gemfile to 3.1.0 (which is what we're using in our application where I was having the problem), and the test started failing. Patching `html_helper.rb` got the test passing again.

So it seems to work in older and newer versions of Redcarpet. If there's anything else you think I should test or any other stylistic problems, let me know.

[compat]: https://github.com/vmg/redcarpet/blob/v1.17.2/lib/redcarpet.rb#L105