Translated named args in if_else()

[markfairbanks]

reprex of issue fixed:

library(data.table)
library(dplyr)
library(dtplyr)

test_df <- data.table(x = c("a", "a", "b", NA))

test_df %>%
  mutate(if_x = if_else(x == "a", true = 1, false = 0, missing = NULL))
#> Error in fifelse(x == "a", true = 1, false = 0, missing = NULL): unused arguments (true = 1, false = 0, missing = NULL)

[markfairbanks]

All set for review

[hadley]

I refactored the tests a little bit — these tests somehow ended up in the wrong place which lead to using show_query() instead of the simpler capture_dot(). I also split out the tests for coalesce since the ifelse tests are reasonably complicated.