Fix column name escaping with test_decoding plugin (#482)

The test_decoding plugin escapes identifiers using double quotes for
column names. However, the pgcopydb transform process also escapes
identifiers in the same way, leading to double escaping and apply process
failures.

This commit addresses the issue by adding double quotes only when the
column name is not already quoted.

Signed-off-by: Arunprasad Rajkumar <ar.arunprasad@gmail.com>