<tsibble>[i] column selection returns <tibble> when key variables are non-syntactic

[mitchelloharawild]

Issue identified from https://github.com/tidyverts/fabletools/issues/350

MRE:

library(tsibble)
#> 
#> Attaching package: 'tsibble'
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, union
names(tourism)[2] <- "My Region"
tourism[1:5]
#> # A tibble: 24,320 x 5
#>    Quarter `My Region` State           Purpose  Trips
#>      <qtr> <chr>       <chr>           <chr>    <dbl>
#>  1 1998 Q1 Adelaide    South Australia Business  135.
#>  2 1998 Q2 Adelaide    South Australia Business  110.
#>  3 1998 Q3 Adelaide    South Australia Business  166.
#>  4 1998 Q4 Adelaide    South Australia Business  127.
#>  5 1999 Q1 Adelaide    South Australia Business  137.
#>  6 1999 Q2 Adelaide    South Australia Business  200.
#>  7 1999 Q3 Adelaide    South Australia Business  169.
#>  8 1999 Q4 Adelaide    South Australia Business  134.
#>  9 2000 Q1 Adelaide    South Australia Business  154.
#> 10 2000 Q2 Adelaide    South Australia Business  169.
#> # i 24,310 more rows
tsibble::tourism[1:5]
#> # A tsibble: 24,320 x 5 [1Q]
#> # Key:       Region, State, Purpose [304]
#>    Quarter Region   State           Purpose  Trips
#>      <qtr> <chr>    <chr>           <chr>    <dbl>
#>  1 1998 Q1 Adelaide South Australia Business  135.
#>  2 1998 Q2 Adelaide South Australia Business  110.
#>  3 1998 Q3 Adelaide South Australia Business  166.
#>  4 1998 Q4 Adelaide South Australia Business  127.
#>  5 1999 Q1 Adelaide South Australia Business  137.
#>  6 1999 Q2 Adelaide South Australia Business  200.
#>  7 1999 Q3 Adelaide South Australia Business  169.
#>  8 1999 Q4 Adelaide South Australia Business  134.
#>  9 2000 Q1 Adelaide South Australia Business  154.
#> 10 2000 Q2 Adelaide South Australia Business  169.
#> # i 24,310 more rows

^{Created on 2024-03-02 with reprex v2.0.2}