Corner case error with plyr::count.

```
> df<-data.frame(a=rep(1:3,5),freq=0)
> plyr::count(df)
Using freq as weighting variable
  a freq freq.1
1 1    0      0
2 2    0      0
3 3    0      0
```

I expected the output:
```
  a freq freq.1
1 1    0      5
2 2    0      5
3 3    0      5
```

`freq` shouldn't be assumed as a weighting variable.