fread colClasses=NA_character_ incorrect

[mattdowle]

Follow up to #1910. Not sure how this lasted so long.

> fread("A,B\n1,3\n2,4\n", colClasses=c(NA_character_, NA_character_))
       A     B      # correct
   <int> <int>
1:     1     3
2:     2     4
> fread("A,B\n1,3\n2,4\n", colClasses=NA_character_)
        A      B    # incorrect
   <char> <char>
1:      1      3
2:      2      4
> 

There's a length-1 specific case at C level which doesn't deal with the NA properly when colClasses is length 1.