Properly handle boolean options in the fields_for helper

[pienkowb]

This PR fixes how boolean options (e.g. inline_errors) are handled in the fields_for helper.

Currently, option defaults are loaded in the helper using the following code:

%i[layout control_col inline_errors label_errors].each do |option|
  field_options[option] ||= options[option]
end

This approach doesn't work for boolean options, because false evalues as falsy, even though it's a legitimate value. As a result, the option value is being overriden with a default value.

For example, the code below does not work as indended (inline errors are still added to the City field):

bootstrap_form_for @user, inline_errors: true do |f|
  f.fields_for :address, inline_errors: false do |af|
    af.text_field :city
  end
end

In order to fix this, I changed:

field_options[option] ||= options[option]

to:

field_options[option] = field_options.key?(option) ? field_options[option] : options[option]

which is more verbose, but works corrently for boolean options.