Fix create_plugin on Windows

Signed-off-by: Joffrey F <joffrey@docker.com>

docker/api/plugin.py

@@ -44,7 +44,10 @@ class PluginApiMixin(object):
         """
         url = self._url('/plugins/create')
 
-        with utils.create_archive(root=plugin_data_dir, gzip=gzip) as archv:
+        with utils.create_archive(
+            root=plugin_data_dir, gzip=gzip,
+            files=set(utils.build.walk(plugin_data_dir, []))
+        ) as archv:
             res = self._post(url, params={'name': name}, data=archv)
         self._raise_for_status(res)
         return True

tests/integration/api_plugin_test.py

@@ -135,7 +135,7 @@ class PluginTest(BaseAPIIntegrationTest):
 
     def test_create_plugin(self):
         plugin_data_dir = os.path.join(
-            os.path.dirname(__file__), 'testdata/dummy-plugin'
+            os.path.dirname(__file__), os.path.join('testdata', 'dummy-plugin')
         )
         assert self.client.create_plugin(
             'docker-sdk-py/dummy', plugin_data_dir