message: [bugfix] handling UTF8 in Druid dimensions
diff: @@ -1318,7 +1318,7 @@ class DruidDatasource(Model, BaseDatasource):
  str instead of an object.
  """
  for col in groupby_cols:
  - df[col] = df[col].fillna('<NULL>').astype(str)
  + df[col] = df[col].fillna('<NULL>').astype('unicode')
  return df
  def query(self, query_obj):
  