Fix Postgres.visitColumn for CASE and RETURNING clauses when column is aliased

When property name for a column different than column name, column is automatically aliased to property name in SELECT clause. Support for CASE clauses in SELECT clause did not enhance Postgres.visitColumn() to not alias columns used within CASE clause.

Postgres.visitColumn() incorrectly handled column aliasing when called during Postgres.visitReturning().

This pull request fixed both issues.