[R-Forge #5572] fread: Column headers with empty strings are skipped in 1.9.2

Submitted by: Roby Joehanes; Assigned to: Nobody; [R-Forge link](https://r-forge.r-project.org/tracker/index.php?func=detail&aid=5572&group_id=240&atid=975)

Suppose the first line of a 5-column flat CSV file is as follows:

``` S
,Col1,Col2,Col3,Col4
```

This line is skipped in `v1.9.2`, and so the data file has no column header (only V1,V2,...,V5). This has not been a problem in `v1.8.10`.
