PDI-9538 - The Update step generate wrong DDL

What was done:
1) Extracted similar method from insert-update step
2) Fixed iisue in the extracted method
3) changed both classes to use extracted method
PDI-9538 - The Update step generate wrong DDL

Added unit test to test the new method
Merge pull request #204 from kolinus/PDI-9538

Pdi 9538 - http://jira.pentaho.com/browse/PDI-9538