Add option to not execute zipped promises

This restores the default behavior for zipped promises and adds an
option (`execute`) to leave them unscheduled.

lib/concurrent/promise.rb

@@ -387,9 +387,13 @@ def flat_map(&block)
     #
     # @return [Promise<Array>]
     def self.zip(*promises)
-      opts = promises.last.is_a?(::Hash) ? promises.pop : {}
+      opts = promises.last.is_a?(::Hash) ? promises.pop.dup : {}
       opts[:executor] ||= ImmediateExecutor.new
-      zero = Promise.new(opts) { [] }
+      zero = if !opts.key?(:execute) || opts.delete(:execute)
+        fulfill([], opts)
+      else
+        Promise.new(opts) { [] }
+      end
 
       promises.reduce(zero) do |p1, p2|
         p1.flat_map do |results|

spec/concurrent/promise_spec.rb

@@ -357,9 +357,21 @@ 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
+      it 'executes the returned Promise by default' do
         composite = promise1.zip(promise2, promise3)
 
+        expect(composite).to be_fulfilled
+      end
+
+      it 'executes the returned Promise when execute is true' do
+        composite = promise1.zip(promise2, promise3, execute: true)
+
+        expect(composite).to be_fulfilled
+      end
+
+      it 'does not execute the returned Promise when execute is false' do
+        composite = promise1.zip(promise2, promise3, execute: false)
+
         expect(composite).to be_unscheduled
       end
 
@@ -389,9 +401,21 @@ 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
+      it 'executes the returned Promise by default' do
         composite = Promise.zip(promise1, promise2, promise3)
 
+        expect(composite).to be_fulfilled
+      end
+
+      it 'executes the returned Promise when execute is true' do
+        composite = Promise.zip(promise1, promise2, promise3, execute: true)
+
+        expect(composite).to be_fulfilled
+      end
+
+      it 'does not execute the returned Promise when execute is false' do
+        composite = Promise.zip(promise1, promise2, promise3, execute: false)
+
         expect(composite).to be_unscheduled
       end