Fix Promise#zip for Promise<Array>

Author: obrok

lib/concurrent/promise.rb

@@ -141,10 +141,10 @@ def flat_map(&block)
     #
     # @return [Promise]
     def zip(*others)
-      others.reduce(self) do |p1, p2|
-        p1.flat_map do |result1|
-          p2.then do |result2|
-            [result1, result2].flatten
+      others.reduce(self.then { |x| [x] }) do |p1, p2|
+        p1.flat_map do |results|
+          p2.then do |next_result|
+            results << next_result
           end
         end
       end

spec/concurrent/promise_spec.rb

@@ -294,12 +294,12 @@ module Concurrent
     describe '#zip' do
       let(:promise1) { Promise.new(executor: executor) { 1 } }
       let(:promise2) { Promise.new(executor: executor) { 2 } }
-      let(:promise3) { Promise.new(executor: executor) { 3 } }
+      let(:promise3) { Promise.new(executor: executor) { [3] } }
 
       it 'yields the results as an array' do
         composite = promise1.zip(promise2, promise3).execute
         sleep 0.1
-        expect(composite.value).to eq([1,2,3])
+        expect(composite.value).to eq([1, 2, [3]])
       end
 
       it 'fails if one component fails' do