The epi_df class is dropped when using training data in bake() #87
Description
As noted in the discussion for the step_training_window() pull request, when using training data in bake() (that is, bake(new_data = NULL) such as in the tests for this function in particular, we now lose the epi_df class of the resulting data frame. In that case, the baked data gets demoted to a plain old tibble.
I think the issue here is that when we put new_data = NULL in bake, bake.recipe sends us to juice() and since the composition is a tibble (by default), then new_data <- tibble::as_tibble(new_data) of juice() is causing the issue (see around line 845 here). This is why I think that all was well when bake(new_data = NULL) was used along with the as_tibble.epi_df function that is no longer in use. So, as Daniel noted, we may have to create our own bake.epi_recipe() and possibly juice.epi_recipe()methods if there is no easier way to solve this.