add option to prepend utf8 bom

- rename add_bom to with_bom
- add spec for with_bom
- change to use expect syntax

lib/comma.rb

@@ -31,12 +31,12 @@ module Comma
     ActionController::Renderers.add :csv do |obj, options|
       filename    = options[:filename]  || 'data'
       extension   = options[:extension] || 'csv'
-
       mime_type = if Rails.version >= '5.0.0'
                     options[:mime_type] || Mime[:csv]
                   else
                     options[:mime_type] || Mime::CSV
                   end
+      with_bom = options.delete(:with_bom) || false
 
       # Capture any CSV optional settings passed to comma or comma specific options
       csv_options = options.slice(*CSV_HANDLER::DEFAULT_OPTIONS.merge(Comma::DEFAULT_OPTIONS).keys)
@@ -50,6 +50,7 @@ module Comma
                end
       end
       data = obj.to_comma(csv_options)
+      data = "\xEF\xBB\xBF#{data}" if with_bom
       disposition = "attachment; filename=\"#{filename}.#{extension}\""
       send_data data, type: mime_type, disposition: disposition
     end

spec/controllers/users_controller_spec.rb

@@ -112,6 +112,18 @@ def get_(name, **args)
           expect(response.content_type).to eq 'text/plain'
         end
 
+        it 'should allow bom to be set' do
+          get :with_custom_options, :format => :csv, :custom_options => { :with_bom => true }
+
+          expected_content =<<-CSV.gsub(/^\s+/,'')
+          \xEF\xBB\xBFFirst name,Last name
+          Fred,Flintstone
+          Wilma,Flintstone
+          CSV
+
+          expect(response.body). to eq expected_content
+        end
+
         describe 'headers' do
           it 'should allow toggling on' do
             get_ :with_custom_options, format: :csv, params: { custom_options: { write_headers: 'true' } }