Fix pulling images with `stream=True`

Pulling an image with option `stream=True` like this:
```
client.api.pull('docker.io/user/repo_name', tag='latest', stream=True)
```
without consuming the generator oftentimes results in premature drop of the connection. Docker daemon tries to send progress of pulling the image to the client, but it encounters an error (broken pipe) and therefore cancells the pull action:
```
Thread 1 "dockerd-dev" received signal SIGPIPE, Broken pipe.
ERRO[2018-09-03T05:12:35.746497638+02:00] Not continuing with pull after error: context canceled
```
As described in issue #2116, even though client receives response with status code 200, image is not pulled.

Closes #2116
Signed-off-by: Przemysław Adamek <adw1n@users.noreply.github.com>

docker/api/image.py

@@ -334,7 +334,8 @@ class ImageApiMixin(object):
         Args:
             repository (str): The repository to pull
             tag (str): The tag to pull
-            stream (bool): Stream the output as a generator
+            stream (bool): Stream the output as a generator. Make sure to
+                consume the generator, otherwise pull might get cancelled.
             auth_config (dict): Override the credentials that
                 :py:meth:`~docker.api.daemon.DaemonApiMixin.login` has set for
                 this request. ``auth_config`` should contain the ``username``

docker/models/images.py

@@ -425,6 +425,7 @@ class ImageCollection(Collection):
         if not tag:
             repository, tag = parse_repository_tag(repository)
 
+        kwargs['stream'] = False
         self.client.api.pull(repository, tag=tag, **kwargs)
         if tag:
             return self.get('{0}{2}{1}'.format(

tests/unit/models_containers_test.py

@@ -232,7 +232,8 @@ class ContainerCollectionTest(unittest.TestCase):
         container = client.containers.run('alpine', 'sleep 300', detach=True)
 
         assert container.id == FAKE_CONTAINER_ID
-        client.api.pull.assert_called_with('alpine', platform=None, tag=None)
+        client.api.pull.assert_called_with('alpine', platform=None, tag=None,
+                                           stream=False)
 
     def test_run_with_error(self):
         client = make_fake_client()

tests/unit/models_images_test.py

@@ -43,7 +43,8 @@ class ImageCollectionTest(unittest.TestCase):
     def test_pull(self):
         client = make_fake_client()
         image = client.images.pull('test_image:latest')
-        client.api.pull.assert_called_with('test_image', tag='latest')
+        client.api.pull.assert_called_with('test_image', tag='latest',
+                                           stream=False)
         client.api.inspect_image.assert_called_with('test_image:latest')
         assert isinstance(image, Image)
         assert image.id == FAKE_IMAGE_ID
@@ -51,7 +52,8 @@ class ImageCollectionTest(unittest.TestCase):
     def test_pull_multiple(self):
         client = make_fake_client()
         images = client.images.pull('test_image')
-        client.api.pull.assert_called_with('test_image', tag=None)
+        client.api.pull.assert_called_with('test_image', tag=None,
+                                           stream=False)
         client.api.images.assert_called_with(
             all=False, name='test_image', filters=None
         )