Can't get 2 or more columns out of DataFrame

When trying to get 2 or more columns out of the DataFrame, only getting the last column

**To Reproduce**
```js
const df = new dfd.DataFrame([{a:1,b:2,c:3},{a:11,b:22,c:33},{a:111,b:222,c:333}]
df['a','b'].print()
```

**Expected behavior**
```js
df['a','b'] 
```
should return a,b columns


