`to_integer()` removes column `spending_time` from the bound table

[jdblischak]

While reviewing #673, I noticed that to_integer() removes the column spending_time from the bound table. Is this a bug? Below is a reprex using the latest commit on main, 732921e:

library("gsDesign2")

x <- gs_design_ahr()
colnames(x$bound)
## [1] "analysis"      "bound"         "probability"   "probability0"  "z"             "~hr at bound" 
## [7] "nominal p"     "spending_time"
xint <- to_integer(x)
colnames(xint$bound)
## [1] "analysis"     "bound"        "probability"  "probability0" "z"            "~hr at bound"
## [7] "nominal p" 

summary(x, display_spending_time = TRUE)
## # A tibble: 1 × 8
## # Groups:   Analysis [1]
##   Analysis Bound     Z `Spending time` `~HR at bound` `Nominal p` `Alternate hypothesis` `Null hypothesis`
##   <chr>    <chr> <dbl>           <dbl>          <dbl>       <dbl>                  <dbl>             <dbl>
## 1 Analysi… Effi…  1.96               1          0.795       0.025                    0.9             0.025

summary(xint, display_spending_time = TRUE)
## Error in `select()` at gsDesign2/R/summary.R:349:3:
## ℹ In argument: `all_of(col_vars)`.
## Caused by error in `all_of()`:
## ! Can't subset elements that don't exist.
## ✖ Element `Spending time` doesn't exist.
## Run `rlang::last_trace()` to see where the error occurred.