spurious column in left outer join

```
from.dfs(equijoin(left.input = to.dfs(mtcars[sample(32, 4),]), right.input = to.dfs(mtcars[sample(32,4),]), outer = "left", map.left = function(k,v) keyval(v$carb,v), map.right = function(k,v) keyval(v$carb, v)))
```

Column is named vr.r or vl.l
