Bugfix regression in value = vars("col") behavior

[yjunechoe]

This PR fixes a regression I introduced in #530, which broke value = vars("col"):

library(pointblank)
df <- data.frame(x = Sys.Date() + 1, y = Sys.Date())

# Works as expected - evaluates to `x > y`
test_col_vals_gt(df, x, vars(y))
#> [1] TRUE

# Fails - evaluates to `x > "y"`
test_col_vals_gt(df, x, vars("y"))
#> [1] NA

Now fixed - both evaluate to the symbol y:

devtools::load_all()
df <- data.frame(x = Sys.Date() + 1, y = Sys.Date())

test_col_vals_gt(df, x, vars(y))
#> [1] TRUE
test_col_vals_gt(df, x, vars("y"))
#> [1] TRUE

Also added a unit test for this equivalence in test-interrogate_simple.R

[rich-iannone]

This is great, thank you!