wrap `balT()` formula arg in `as.formula()`

[benthestatistician]

In an example I'm working with I got

> covariates %>% setdiff("adjcnt_a20_64") %>%
+     paste(collapse="+") %>% paste("z ~", .) %>% 
+ balanceTest(counties, unit.weights=adjcnt_a20_64)
Error: $ operator is invalid for atomic vectors

Was able to get it to run by

covariates %>% setdiff("adjcnt_a20_64") %>%
    paste(collapse="+") %>% paste("z ~", .) %>%
    as.formula() %>%
balanceTest(counties, unit.weights=adjcnt_a20_64)

Perhaps this is something we can take care of rather than leaving it to the user to figure out? (proj1-balT@ec7b1ee)

[josherrickson]

Seems this would fit into a larger solution to dispatch on the first argument of balanceTest to make balanceTest.formula and balanceTest.character.