Add column names method

[hatsu38]

This pull request adds the column_names method to the ActiveHash gem.

The column_names method is analogous to the one found in Rails' ActiveRecord, allowing users to retrieve the names of columns defined in an ActiveHash model.

Added column_names method to ActiveHash::Base class.
The method returns an array of string column names, similar to ActiveRecord's column_names method.

Country.column_names
# => ["id", "name", "code"]

close #310

[flavorjones]

@hatsu38 Thank you for submitting this!

If the goal here is to make sure active hash integrates with the CSV gem, can you please add a new spec that verifies this is working as expected? I'd suggest creating a new directory, spec/integration, and a new spec file.

I would also be interested in a short "how to generate CSV" in the README.md file.

[flavorjones]

I would also like to see a short explicit test in spec/active_hash/base_spec.rb like the one I added in #312 for #field_names. Thank you!

[kbrock]

Rails does not have field_names, instead they have column_names. So there is a difference.
Rails explicitly states that column_names is an array of strings.

I'd feel more comfortable with just doing the work for column_names.
@flavorjones what is your take?

def column_names
  field_names.map(&:name)
end

Also looks like field_names << field is not ensuring that a symbol goes in there, but comparison routines assume it is a symbol. I'll create a PR for that.

[flavorjones]

@kbrock I agree, let's not cache column names, and always iterate over field names when it's called. @hatsu38 Can you make that change?

[kbrock]

Now field names are all symbols, so I think we can simplify this:

Suggested change

	field_names.map do |field|
	field.respond_to?(:name) ? field.name : field.to_s.freeze
	end
	fields.map(&:name)

Does that work for you?

[hatsu38]

After merging the latest master, the code worked perfectly! 0306858