Coerce into tibbles

[howardbaik]

Some functions output data frames that contain unnecessary row names and fill up the console, which makes it hard to look at. Instead, let's output tibbles using tibble::as_tibble().

googlesheets4 does this here. That line makes the spread_sheet_impl_() spit out a tibble, and spread_sheet_impl_() is used inside range_read() (a.k.a read_sheet())