Translate column name pi into PI()

[herbhuang]

When select/rename column named pi, query is back translate to PI() in SQL and return 3.14...

Source: backend.R

require(dbplyr)
require(dplyr)

mtcarss <- mtcars
mtcarss$pi  <- mtcars$mpg * 10
mtcars_db <- tbl_memdb(mtcarss)
mtcars_db %>% 
  select(mpg, cyl, "pi") %>%
  select(mpg, pi) %>%
  rename(pretax_income = pi) %>%
  show_query()