Nit: head() and tail() puke if less than rows than specified (default=5)

If my DataFrame just has 2 rows, and I call `df.head()`, I get:

```
Uncaught Error: ParamError: Number of rows cannot be greater than available rows in data
    at DataFrame.head (/.../danfojs-node/dist/danfojs-base/core/frame.js:553:19)
```

That is not how Pandas behaves.  I think just my 2 rows should be returned.