asyncio.tasks: Make sure CoroWrapper.send proxies one argument correctly

Issue #21209.

Lib/asyncio/tasks.py

@@ -53,6 +53,8 @@ class CoroWrapper:
         # We use `*value` because of a bug in CPythons prior
         # to 3.4.1. See issue #21209 and test_yield_from_corowrapper
         # for details.  This workaround should be removed in 3.5.0.
+        if len(value) == 1:
+            value = value[0]
         return self.gen.send(value)
 
     def throw(self, exc):

Lib/test/test_asyncio/test_tasks.py

@@ -1410,6 +1410,24 @@ class TaskTests(unittest.TestCase):
         finally:
             asyncio.tasks._DEBUG = old_debug
 
+    def test_yield_from_corowrapper_send(self):
+        def foo():
+            a = yield
+            return a
+
+        def call(arg):
+            cw = asyncio.tasks.CoroWrapper(foo(), foo)
+            cw.send(None)
+            try:
+                cw.send(arg)
+            except StopIteration as ex:
+                return ex.args[0]
+            else:
+                raise AssertionError('StopIteration was expected')
+
+        self.assertEqual(call((1, 2)), (1, 2))
+        self.assertEqual(call('spam'), 'spam')
+
 
 class GatherTestsBase: