Do not execute zipped promises

It's unnecessary to immediately execute promises that are chained
together with `zip`. This feels like a bug to me, but I'm sure people
are relying on the old behavior, so this is definitely a breaking
change.

Author: davishmcclurg

lib/concurrent/promise.rb

@@ -387,7 +387,7 @@ def flat_map(&block)
     #
     # @return [Promise<Array>]
     def self.zip(*promises)
-      zero = fulfill([], executor: ImmediateExecutor.new)
+      zero = Promise.new(executor: ImmediateExecutor.new) { [] }
 
       promises.reduce(zero) do |p1, p2|
         p1.flat_map do |results|

spec/concurrent/promise_spec.rb

@@ -357,6 +357,12 @@ def get_ivar_from_args(opts)
       let(:promise2) { Promise.new(executor: :immediate) { 2 } }
       let(:promise3) { Promise.new(executor: :immediate) { [3] } }
 
+      it 'does not execute the returned Promise' do
+        composite = promise1.zip(promise2, promise3)
+
+        expect(composite).to be_unscheduled
+      end
+
       it 'yields the results as an array' do
         composite = promise1.zip(promise2, promise3).execute.wait
 
@@ -375,6 +381,12 @@ def get_ivar_from_args(opts)
       let(:promise2) { Promise.new(executor: :immediate) { 2 } }
       let(:promise3) { Promise.new(executor: :immediate) { [3] } }
 
+      it 'does not execute the returned Promise' do
+        composite = Promise.zip(promise1, promise2, promise3)
+
+        expect(composite).to be_unscheduled
+      end
+
       it 'yields the results as an array' do
         composite = Promise.zip(promise1, promise2, promise3).execute.wait