[SPARK-20474] Fixing OnHeapColumnVector reallocation

## What changes were proposed in this pull request?
OnHeapColumnVector reallocation copies to the new storage data up to 'elementsAppended'. This variable is only updated when using the ColumnVector.appendX API, while ColumnVector.putX is more commonly used.

## How was this patch tested?
Tested using existing unit tests.

Author: Michal Szafranski <michal@databricks.com>

Closes #17773 from michal-databricks/spark-20474.