Return source code with quotes unified.

def _format_code(source, preferred_quote):
    """Return source code with quotes unified."""
    if not source:
        return source

    modified_tokens = []

    sio = io.StringIO(source)
    for (token_type,
         token_string,
         start,
         end,
         line) in tokenize.generate_tokens(sio.readline):

        if token_type == tokenize.STRING:
            token_string = unify_quotes(token_string,
                                        preferred_quote=preferred_quote)

        modified_tokens.append(
            (token_type, token_string, start, end, line))

    return untokenize.untokenize(modified_tokens)