Reload missing properties before download.

Fixes #1555.

gcloud/storage/blob.py

@@ -287,6 +287,9 @@ def download_to_file(self, file_obj, client=None):
         :raises: :class:`gcloud.exceptions.NotFound`
         """
         client = self._require_client(client)
+        if self.media_link is None:  # not yet loaded
+            self.reload()
+
         download_url = self.media_link
 
         # Use apitools 'Download' facility.

gcloud/storage/test_blob.py

@@ -291,6 +291,32 @@ def test_delete(self):
         self.assertFalse(blob.exists())
         self.assertEqual(bucket._deleted, [(BLOB_NAME, None)])
 
+    def test_download_to_file_wo_media_link(self):
+        from six.moves.http_client import OK
+        from six.moves.http_client import PARTIAL_CONTENT
+        from io import BytesIO
+        BLOB_NAME = 'blob-name'
+        MEDIA_LINK = 'http://example.com/media/'
+        chunk1_response = {'status': PARTIAL_CONTENT,
+                           'content-range': 'bytes 0-2/6'}
+        chunk2_response = {'status': OK,
+                           'content-range': 'bytes 3-5/6'}
+        connection = _Connection(
+            (chunk1_response, b'abc'),
+            (chunk2_response, b'def'),
+        )
+        # Only the 'reload' request hits on this side:  the others are done
+        # through the 'http' object.
+        reload_response = {'status': OK, 'content-type': 'application/json'}
+        connection._responses = [(reload_response, {"mediaLink": MEDIA_LINK})]
+        client = _Client(connection)
+        bucket = _Bucket(client)
+        blob = self._makeOne(BLOB_NAME, bucket=bucket)
+        fh = BytesIO()
+        blob.download_to_file(fh)
+        self.assertEqual(fh.getvalue(), b'abcdef')
+        self.assertEqual(blob.media_link, MEDIA_LINK)
+
     def _download_to_file_helper(self, chunk_size=None):
         from six.moves.http_client import OK
         from six.moves.http_client import PARTIAL_CONTENT