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.

util/bytes.py

@@ -0,0 +1,32 @@
+class Bytes(object):
+  """ Wrapper around strings and unicode objects to ensure we are always using
+      the correct encoded or decoded data.
+  """
+  def __init__(self, data):
+    assert isinstance(data, str)
+    self._encoded_data = data
+
+  @classmethod
+  def for_string_or_unicode(cls, input):
+    # If the string is a unicode string, then encode its data as UTF-8. Note that
+    # we don't catch any decode exceptions here, as we want those to be raised.
+    if isinstance(input, unicode):
+      return Bytes(input.encode('utf-8'))
+
+    # Next, try decoding as UTF-8. If we have a utf-8 encoded string, then we have no
+    # additional conversion to do.
+    try:
+      input.decode('utf-8')
+      return Bytes(input)
+    except UnicodeDecodeError:
+      pass
+
+    # Finally, if the data is (somehow) a unicode string inside a `str` type, then
+    # re-encoded the data.
+    return Bytes(input.encode('utf-8'))
+
+  def as_encoded_str(self):
+    return self._encoded_data
+
+  def as_unicode(self):
+    return self._encoded_data.decode('utf-8')