Fix handling of manifests with unicode in the backfill

Also adds a bunch of tests around manifests to ensure we get the same information in and out

test/registry/registry_tests.py

@@ -66,6 +66,25 @@ def test_basic_push_pull_by_manifest(manifest_protocol, basic_images, liveserver
                          credentials=credentials)
 
 
+def test_basic_push_by_manifest_digest(manifest_protocol, basic_images, liveserver_session,
+                                       app_reloader):
+  """ Test: Basic push-by-manifest and pull-by-manifest of an image to a new repository. """
+  credentials = ('devtable', 'password')
+
+  # Push a new repository.
+  options = ProtocolOptions()
+  options.push_by_manifest_digest = True
+
+  result = manifest_protocol.push(liveserver_session, 'devtable', 'newrepo', 'latest', basic_images,
+                                  credentials=credentials, options=options)
+
+  # Pull the repository by digests to verify.
+  digests = [str(manifest.digest) for manifest in result.manifests.values()]
+  manifest_protocol.pull(liveserver_session, 'devtable', 'newrepo', digests, basic_images,
+                         credentials=credentials)
+
+
+
 def test_push_invalid_credentials(pusher, basic_images, liveserver_session, app_reloader):
   """ Test: Ensure we get auth errors when trying to push with invalid credentials. """
   invalid_credentials = ('devtable', 'notcorrectpassword')