enable inserting NULL to object type columns

This fixes #310 by allowing inserting NULL to object type such as MDSYS.SDO_GEOMETRY.
This patch works on jruby and latest ruby-oci8 in git master, which will be released as 2.1.6.

Note that this enables inserting NULL only. Not-null values cannot be inserted because they are converted to `String` by `YAML#dump` in `ActiveRecord::ConnectionAdapters::Quoting#type_cast`.