na.strings may fail to convert numbers into NAs in dev

This case is working :
```
> fread("A,B\n1987,1\n1987,3\n", na.strings=c("1987", "NA"))
    A B
1: NA 1
2: NA 3
```
this one isn't in dev:
```
> fread("A,B\n1987,1\n1,3\n", na.strings=c("1987", "NA"))
      A B
1: 1987 1
2:    1 3
```
It's ok on CRAN.