perf(storage): blob downloads gzipped content.

GET requests now contain the header accept-encoding:gzip
This improves performance of compressible strings which
were uploaded as gzips.

The caller is not required to do any decompression
because decompressiong is handle by the library.

Confusing `IOError: CRC check failed` exceptions will be
risen in the case of mid-download connection breaks.

storage/google/cloud/storage/blob.py

@@ -462,6 +462,7 @@ def download_to_file(self, file_obj, client=None):
         """
         download_url = self._get_download_url()
         headers = _get_encryption_headers(self._encryption_key)
+        headers['accept-encoding'] = 'gzip'
         transport = self._make_transport(client)
 
         try:

storage/tests/unit/test_blob.py

@@ -405,6 +405,7 @@ def _check_session_mocks(self, client, fake_session_factory,
         #       **MUTABLE** headers and it was mutated before the
         #       second request.
         headers['range'] = 'bytes=3-5'
+        headers['accept-encoding'] = 'gzip'
         call = mock.call(
             'GET', expected_url, data=None, headers=headers)
         self.assertEqual(fake_transport.request.mock_calls, [call, call])
@@ -493,7 +494,8 @@ def test_download_to_file_with_failure(self, fake_session_factory):
         fake_session_factory.assert_called_once_with(client._credentials)
         # Check that the transport was called once.
         transport.request.assert_called_once_with(
-            'GET', blob.media_link, data=None, headers={})
+            'GET', blob.media_link, data=None,
+            headers={'accept-encoding':'gzip'})
 
     @mock.patch('google.auth.transport.requests.AuthorizedSession')
     def test_download_to_file_wo_media_link(self, fake_session_factory):
@@ -553,7 +555,8 @@ def _download_to_file_helper(self, fake_session_factory, use_chunks=False):
             # Check that exactly one transport was created.
             fake_session_factory.assert_called_once_with(client._credentials)
             fake_transport.request.assert_called_once_with(
-                'GET', media_link, data=None, headers={})
+                'GET', media_link, data=None,
+                headers={'accept-encoding': 'gzip'})
 
     def test_download_to_file_default(self):
         self._download_to_file_helper()
@@ -645,6 +648,7 @@ def test_download_to_filename_w_key(self, fake_session_factory):
             'X-Goog-Encryption-Key-Sha256': header_key_hash_value,
             'X-Goog-Encryption-Algorithm': 'AES256',
             'X-Goog-Encryption-Key': header_key_value,
+            'accept-encoding':'gzip'
         }
         self._check_session_mocks(
             client, fake_session_factory, media_link, headers=key_headers)