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ARROW-5190 [R]: Discussion: tibble dependency in R package

`tibble` is now on Suggests. The code still makes tibbles, so that if `tibble` is otherwise loaded, data frames are nicely print, etc... but does not need to specifically `Imports`  it.

Author: Romain Francois <romain@rstudio.com>

Closes #4454 from romainfrancois/ARROW-5190/tibble and squashes the following commits:

18b1e05e <Romain Francois> s/as_tibble/as.data.frame/ in tests
66e63038 <Romain Francois> as.data.frame() needs arguments row.names = NULL, optional = FALSE
240ef90d <Romain Francois> move tibble dependency to Suggests