Further fixes for unicode handling in manifests

We were occasionally trying to compute schema 2 version 1 signatures on the *unicode* representation, which was failing the signature check. This PR adds a new wrapper type called `Bytes`, which all manifests must take in, and which handles the unicodes vs encoded utf-8 stuff in a central location. This PR also adds a test for the manifest that was breaking in production.

image/docker/schema2/config.py

@@ -102,7 +102,7 @@ from dateutil.parser import parse as parse_date
 
 from digest import digest_tools
 from image.docker import ManifestException
-from image.docker.schemautil import ensure_utf8
+from util.bytes import Bytes
 
 
 DOCKER_SCHEMA2_CONFIG_HISTORY_KEY = "history"
@@ -183,10 +183,12 @@ class DockerSchema2Config(object):
   }
 
   def __init__(self, config_bytes):
+    assert isinstance(config_bytes, Bytes)
+
     self._config_bytes = config_bytes
 
     try:
-      self._parsed = json.loads(ensure_utf8(config_bytes))
+      self._parsed = json.loads(config_bytes.as_unicode())
     except ValueError as ve:
       raise MalformedSchema2Config('malformed config data: %s' % ve)
 
@@ -198,12 +200,12 @@ class DockerSchema2Config(object):
   @property
   def digest(self):
     """ Returns the digest of this config object. """
-    return digest_tools.sha256_digest(ensure_utf8(self._config_bytes))
+    return digest_tools.sha256_digest(self._config_bytes.as_encoded_str())
 
   @property
   def size(self):
     """ Returns the size of this config object. """
-    return len(ensure_utf8(self._config_bytes))
+    return len(self._config_bytes.as_encoded_str())
 
   @property
   def bytes(self):