Converting all GROUPBY columns to strings

        When grouping by a numeric (say FLOAT) column, pydruid returns
        strings in the dataframe. This creates issues downstream related
        to having mixed types in the dataframe

        Here we replace None with <NULL> and make the whole series a
        str instead of an object.

def homogenize_types(df, groupby_cols):
        """Converting all GROUPBY columns to strings

        When grouping by a numeric (say FLOAT) column, pydruid returns
        strings in the dataframe. This creates issues downstream related
        to having mixed types in the dataframe

        Here we replace None with <NULL> and make the whole series a
        str instead of an object.
        """
        for col in groupby_cols:
            df[col] = df[col].fillna('<NULL>').astype('unicode')
        return df