message: [qa] allow passing headers to msg_headers
diff: @@ -1600,8 +1600,8 @@ class msg_headers(object):
  """
  command = b"headers"
  - def __init__(self):
  - self.headers = []
  + def __init__(self, headers = None):
  + self.headers = [] if headers is None else headers
  def deserialize(self, f):
  # comment in bitcoind indicates these should be deserialized as blocks
  