fix typo filter param for `volumes`

https://docs.docker.com/engine/reference/api/docker_remote_api_v1.21/#list-volumesSigned-off-by: Nicolas Delaby <nicolas.delaby@lock8.me>

docker/api/volume.py

@@ -5,7 +5,7 @@ class VolumeApiMixin(object):
     @utils.minimum_version('1.21')
     def volumes(self, filters=None):
         params = {
-            'filter': utils.convert_filters(filters) if filters else None
+            'filters': utils.convert_filters(filters) if filters else None
         }
         url = self._url('/volumes')
         return self._result(self._get(url, params=params), True)

tests/unit/volume_test.py

@@ -17,6 +17,18 @@ class VolumeTest(DockerClientTest):
         self.assertEqual(args[0][0], 'GET')
         self.assertEqual(args[0][1], url_prefix + 'volumes')
 
+    @base.requires_api_version('1.21')
+    def test_list_volumes_and_filters(self):
+        volumes = self.client.volumes(filters={'dangling': True})
+        assert 'Volumes' in volumes
+        assert len(volumes['Volumes']) == 2
+        args = fake_request.call_args
+
+        assert args[0][0] == 'GET'
+        assert args[0][1] == url_prefix + 'volumes'
+        assert args[1] == {'params': {'filters': '{"dangling": ["true"]}'},
+                           'timeout': 60}
+
     @base.requires_api_version('1.21')
     def test_create_volume(self):
         name = 'perfectcherryblossom'