Implement possibility of handling of NUMBER columns as :float

Adds new `ActiveRecord::ConnectionAdapters::OracleEnhancedAdapter.number_datatype_coercion` class attribute preference

Makes change to handling of virtual NUMBER columns in schema dumps

Adds RSpec test for simulated db:schema:dump & db:test:load scenario amongst others.
Has some small cleanup & syntax improvement changes.

Disclaimer:
  This commit includes Benjamin Ortega's (github: substars) code from https://github.com/rsim/oracle-enhanced/pull/258

Closes: rsim/oracle-enhanced#258

Signed-off-by: Aleksandrs Ļedovskis <aleksandrs@ledovskis.lv>
Merge pull request #418 from aledovsky/bug/float_column_type

Implement handling of NUMBER columns as :float