'define_group' ignores 'format' config

[kurko]

What

When define_group is present, format is ignored.

Example

In case the following example, it works as expected. The format used is api_blueprint (index.apib file, from this PR) and the docs go to doc/.

RspecApiDocumentation.configure do |config|
  config.filter = :public
  config.format = :api_blueprint
end

However, I want a different directory, so I do the following:

RspecApiDocumentation.configure do |config|
  config.define_group :public do |group|
    group.docs_dir = Rails.root.join("docs", "api")
    group.filter = :public
  end
  config.format = :api_blueprint
end

In this case, docs go to docs/api/, but it ignores format. The final file is an index.html (default format).