Added method to return array of instance_methods_as_symbols.

- In the ActiveRecord adapter, we want to let ActiveRecord call
  define_attribute_methods. If we call it, we're calling it too early, which
  causes the wrong accessor methods to be created. This was the problem
  that was being encountered with serialize. That problem is documented
  here
  https://github.com/attr-encrypted/attr_encrypted/commit/0b8c0f6979be8ab5e8fbff184e4fde2fa4666fd3
- By letting ActiveRecord define accessor methods, things behave as they
  should. Namely, ActiveRecord checks to see if an attribute already has
  accessor methods defined before defining them. Using something like
  serialize, accessors are written, and then ActiveRecord sees this and
  doesn't define the accessor methods. So the attr_encrypted and
  serialize accessors work as expected, as well as normal accessors.
- We add the column names to the instance_methods_as_symbols array so
  that attr_encrypted will not define accessor methods for the columns,
  we let ActiveRecord do this for us, as it should.