omit trailing comma with collapse operation

Before

```
$ groupBy -i ex1.out -g 1,2,3,4 -c 8,9 -o collapse,mean
chr1 10  20  A   B.1,B.2,    5500
```

After

```
$ groupBy -i ex1.out -g 1,2,3,4 -c 8,9 -o collapse,mean
chr1 10  20  A   B.1,B.2    5500
```