make augment() return tibbles

[EmilHvitfeldt]

This PR makes augment() return tibbles even if the input data is just a data.frame

library(parsnip)

lm_spec <- linear_reg() %>%
  set_engine("lm")

lm_fit <- lm_spec %>% fit(mpg ~ ., data = mtcars)

class(mtcars)
#> [1] "data.frame"

augment(lm_fit, new_data = mtcars) %>% class()
#> [1] "tbl_df"     "tbl"        "data.frame"

^{Created on 2021-04-23 by the reprex package (v1.0.0)}

To close #468

[juliasilge]

Thank you so much @EmilHvitfeldt! 🙌

[github-actions]

This pull request has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex: https://reprex.tidyverse.org) and link to this issue.