Set columns to their DEFAULT values by default

The SQL INSERT command does not have to provide values for all columns in a table. Those columns that are omitted will get their DEFAULT values.

To achieve such decoupling from the table schema, a Bulk Write SDK application has to:

1) retrieve DEFAULT column value using getDefaultValue()
2) call SetValue() to "fill" the column value with its DEFAULT

This is necessary because current Bulk Write SDK requires all columns to be "filled," otherwise the bulk->writeRow() call raises an error: Not all the columns for this row have been filled.

To improve Bulk Write SDK applications, I propose eliminating the need to "fill" columns with their DEFAULT values using the SetValue() call for each row.