Use shlex for exec string cmd, added integration testcase.

docker/client.py

@@ -548,8 +548,8 @@ class Client(requests.Session):
             raise Exception('Exec is not supported in API < 1.15!')
         if isinstance(container, dict):
             container = container.get('Id')
-        if not isinstance(cmd, (list, tuple)):
-            cmd = [cmd]
+        if isinstance(cmd, six.string_types):
+            cmd = shlex.split(str(cmd))
 
         data = {
             'Container': container,

tests/integration_test.py

@@ -630,7 +630,8 @@ class TestRestartingContainer(BaseTestCase):
 
 class TestExecuteCommand(BaseTestCase):
     def runTest(self):
-        container = self.client.create_container('busybox', ['false'])
+        container = self.client.create_container('busybox', 'cat',
+                                                 detach=True, stdin_open=True)
         id = container['Id']
         self.client.start(id)
         self.tmp_containers.append(id)
@@ -640,9 +641,23 @@ class TestExecuteCommand(BaseTestCase):
         self.assertEqual(res, expected)
 
 
+class TestExecuteCommandString(BaseTestCase):
+    def runTest(self):
+        container = self.client.create_container('busybox', 'cat',
+                                                 detach=True, stdin_open=True)
+        id = container['Id']
+        self.client.start(id)
+        self.tmp_containers.append(id)
+
+        res = self.client.execute(id, 'echo hello world', stdout=True)
+        expected = b'hello world\n' if six.PY3 else 'hello world\n'
+        self.assertEqual(res, expected)
+
+
 class TestExecuteCommandStreaming(BaseTestCase):
     def runTest(self):
-        container = self.client.create_container('busybox', ['false'])
+        container = self.client.create_container('busybox', 'cat',
+                                                 detach=True, stdin_open=True)
         id = container['Id']
         self.client.start(id)
         self.tmp_containers.append(id)