Compatibility with dev dbplyr

[mgirlich]

The dev version of dbplyr requires that the con argument of translate_sql() must not be NULL. This breaks one of your tests in test-db_timestamp.R:

expect_identical(
  db_timestamp.default(ts_posix, conn = NULL),
  db_timestamp.default(ts_str, conn = NULL)
)

I don't think it makes much sense to simply allow con = NULL, as this might lead to unexpected translations. Which is why the con argument now must not be NULL. It would be great if you could fix this so that we can release dbplyr to CRAN soon.

[marcusmunch]

Good point; if we're calling translate_sql(), it makes sense that we would require con to actually be a connection object.

The fix should be as simple as adding db_timestamp.NULL method, so that calls to translate_sql will never be NULL. I'm working on it right now.