[SPARK-18823][SPARKR] add support for assigning to column

## What changes were proposed in this pull request?

Support for
```
df[[myname]] <- 1
df[[2]] <- df$eruptions
```

## How was this patch tested?

manual tests, unit tests

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16663 from felixcheung/rcolset.