docs: def to_utf8(value):
    """Returns a string encoded using UTF-8.
  
    This function comes from `Tornado`_.
  
    :param value:
      A unicode or string to be encoded.
    :returns:
      The encoded string.
    """
    if isinstance(value, unicode):
      return value.encode('utf-8')
  
    assert isinstance(value, str)
    return value