Fix combined experiments (#502)

1. always pass all alternatives to prevent unintentional version bumps
2. return the chosen alternative from `ab_combined_test`

Fixes #500

lib/split/combined_experiments_helper.rb

@@ -6,6 +6,7 @@ def ab_combined_test(metric_descriptor, control = nil, *alternatives)
       raise(Split::InvalidExperimentsFormatError, 'Unable to find experiment #{metric_descriptor} in configuration') if experiment[:combined_experiments].nil?
 
       alternative = nil
+      weighted_alternatives = nil
       experiment[:combined_experiments].each do |combined_experiment|
         if alternative.nil?
           if control
@@ -15,9 +16,15 @@ def ab_combined_test(metric_descriptor, control = nil, *alternatives)
             alternative = ab_test(combined_experiment, normalized_alternatives[0], *normalized_alternatives[1])
           end
         else
-          ab_test(combined_experiment, [{alternative => 1}])
+          weighted_alternatives ||= experiment[:alternatives].each_with_object({}) do |alt, memo|
+            alt = Alternative.new(alt, experiment[:name]).name
+            memo[alt] = (alt == alternative ? 1 : 0)
+          end
+
+          ab_test(combined_experiment, [weighted_alternatives])
         end
       end
+      alternative
     end
 
     def find_combined_experiment(metric_descriptor)

spec/combined_experiments_helper_spec.rb

@@ -46,12 +46,12 @@
       end
     end
 
-    it "uses same alternatives for all sub experiments " do
+    it "uses same alternative for all sub experiments and returns the alternative" do
       allow(self).to receive(:get_alternative) { "test-alt" }
       expect(self).to receive(:ab_test).with(:exp_1_click, {"control"=>0.5}, {"test-alt"=>0.5}) { "test-alt" }
-      expect(self).to receive(:ab_test).with(:exp_1_scroll, [{"test-alt" => 1}] )
+      expect(self).to receive(:ab_test).with(:exp_1_scroll, [{"control" => 0, "test-alt" => 1}])
 
-      ab_combined_test('combined_exp_1')
+      expect(ab_combined_test('combined_exp_1')).to eq('test-alt')
     end
   end
 end