[SPARKR] fix warnings

## What changes were proposed in this pull request?

Fix for a bunch of test warnings that were added recently.
We need to investigate why warnings are not turning into errors.

```
Warnings -----------------------------------------------------------------------
1. createDataFrame uses files for large objects (test_sparkSQL.R#215) - Use Sepal_Length instead of Sepal.Length  as column name

2. createDataFrame uses files for large objects (test_sparkSQL.R#215) - Use Sepal_Width instead of Sepal.Width  as column name

3. createDataFrame uses files for large objects (test_sparkSQL.R#215) - Use Petal_Length instead of Petal.Length  as column name

4. createDataFrame uses files for large objects (test_sparkSQL.R#215) - Use Petal_Width instead of Petal.Width  as column name

Consider adding
  importFrom("utils", "object.size")
to your NAMESPACE file.
```

## How was this patch tested?

unit tests

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #15560 from felixcheung/rwarnings.