Read type from column_type besides type for better supporting belongs_to

[lurodrigo]

Following up on the discussion on #36

The PR intends to better support belongs_to.

To get the type, we first read column_type from opts, and only if it's not available we read type from opts. This way, we can use column_type everywhere if we want to, but we can also keep using type and not break existing code.

For belongs_to, the user can use belongs_to(:post, Post, type: TypeID) to use the default type for the column_type or specify it: belongs_to(:post, Post, type: TypeID, column_type: :string)

My comment here describes a reproducible example of the issue and its solution #36 (comment)