Do not overwrite the database default value if provided by attributes

[reuben453]

When a new database record has a column that is set to its database default value, the "#{attribute}_changed?" method returns false, which causes the connection_default_value_defined value to be true and causes default_value_for to overwrite the database default.

This PR changes this behavior to not overwrite when the column is being set by the initialization attributes. It will continue to overwrite the database default if the attribute is not present in the initialization attributes.

Example code:

ActiveRecord::Base.connection.create_table(:books, :force => true) do |t|
  t.integer :count, :null => false, :default => 1
end

Book.default_value_for(:count, :allows_nil => false) { 1234 }

Book.new(count: 1).count # This should return `1`. Currently, this returns `1234`. This PR fixes this.
Book.new.count # This should continue to return `1234`.

[reuben453]

I noticed that this case did not have a test, so added one here.

[reuben453]

@chrisarcand Would you mind having a look at these changes please? 🙏

[jrafanie]

@reuben453 sorry you haven't had an update on this PR. I'm now maintaining it in my free time. Is this still an issue for you? I see the test does still fail when I run master code against it.

I don't think the intention was to use column defaults with default_value_for. When I tested column defaults, it's great if it's supported in your database but it's limited in it's usage. On the other hand, it's in the database so likely more performant than doing things in ruby. I see you're intention was to use both but the default from the attributes should "win". I don't know if that's a good idea for this gem. I could be convinced though if you have use case to use both attribute defaults and default_value_for. My instincts say we should use attribute defaults where it works for us and use default_value_for to fill an use cases missing in attribute defaults.

I'll keep this open for a week but I think I'll close it unless we can better describe good use cases for using both sets of defaults. Thanks!