Reset @column_defaults when assigning .

Reset @column_defaults when assigning `locking_column`.
We had a potential problem. For example:

```ruby
class Post < ActiveRecord::Base
  self.column_defaults  # if we call this unintentionally before setting locking_column ...
  self.locking_column = 'my_locking_column'
end

Post.column_defaults["my_locking_column"]
=> nil # expected value is 0 !
```

The cause of this problem is @column_defaults caching.

```ruby
# activerecrd/lib/active_record/locking/optimistic.rb
def column_defaults
  @column_defaults ||= begin
    defaults = super

    if defaults.key?(locking_column) && lock_optimistically
      defaults[locking_column] ||= 0
    end

    defaults
  end
end
```