Error in logic for column name detection

[st-pasha]

> fread("A,100,200\n,300,400\n,500,600")
    A 100 200
1: NA 300 400
2: NA 500 600

A better guess here would be that all rows are data...

[st-pasha]

The issue has reappeared in latest data.table:

> fread("A,100,200\n,300,400\n,500,600")
        A   100   200
   <lgcl> <int> <int>
1:     NA   300   400
2:     NA   500   600