Change spec to test Promise in both side

spec/api-ipc-spec.coffee

@@ -52,11 +52,10 @@ describe 'ipc module', ->
       print_name = remote.require path.join(fixtures, 'module', 'print_name.js')
       assert.equal print_name.print(buf), 'Buffer'
 
-  describe 'remote promise in renderer', ->
-    it 'can be used as promise', (done) ->
+  describe 'remote promise', ->
+    it 'can be used as promise in each side', (done) ->
       promise = remote.require path.join(fixtures, 'module', 'promise.js')
-      promise.toPromise(1234)
-        .then (value) => value * 2
+      promise.twicePromise(Promise.resolve(1234))
         .then (value) =>
           assert.equal value, 2468
           done()

spec/fixtures/module/promise.js

@@ -1,3 +1,5 @@
-exports.toPromise = function (value) {
-  return Promise.resolve(value);
-};
+exports.twicePromise = function (promise) {
+  return promise.then(function (value) {
+    return value * 2;
+  });
+}