'file' key can be omitted from extends

If the 'file' key is not set in the extends_options dict then we
look for the 'service' from within the same file.

Fixes this issue: https://github.com/docker/compose/issues/1237

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

tests/fixtures/extends/no-file-specified.yml

@@ -0,0 +1,9 @@
+myweb:
+  extends:
+    service: web
+  environment:
+    - "BAR=1"
+web:
+  image: busybox
+  environment:
+    - "BAZ=3"

tests/unit/config_test.py

@@ -447,6 +447,30 @@ class ExtendsTest(unittest.TestCase):
         dictionary['extends']['what'] = 'is this'
         self.assertRaisesRegexp(config.ConfigurationError, 'what', load_config)
 
+    def test_extends_file_defaults_to_self(self):
+        """
+        Test not specifying a file in our extends options that the
+        config is valid and correctly extends from itself.
+        """
+        service_dicts = config.load('tests/fixtures/extends/no-file-specified.yml')
+        self.assertEqual(service_dicts, [
+            {
+                'name': 'myweb',
+                'image': 'busybox',
+                'environment': {
+                    "BAR": "1",
+                    "BAZ": "3",
+                }
+            },
+            {
+                'name': 'web',
+                'image': 'busybox',
+                'environment': {
+                    "BAZ": "3",
+                }
+            }
+        ])
+
     def test_blacklisted_options(self):
         def load_config():
             return config.make_service_dict('myweb', {