fread should properly handle NA in colClasses argument

[jangorecki]

str(data.table::fread("a,b\n1,a", colClasses=c(NA, "factor")))
#Classes ‘data.table’ and 'data.frame':	1 obs. of  2 variables:
# $ a: Factor w/ 1 level "1": 1
# $ b: Factor w/ 1 level "a": 1
str(read.csv(textConnection("a,b\n1,a"), colClasses=c(NA, "factor")))
#'data.frame':	1 obs. of  2 variables:
# $ a: int 1
# $ b: Factor w/ 1 level "a": 1

read.table
scan.R

also an issue with mixed character and factor results was fixed:

fread("a,b\n1,a", colClasses=c("character", "factor"))