Changed expected value quotation for assert_same_exception to heredoc.
Usage:
    assert_same_exception(<<-END)
        <Exception Class>: <Exception Message>
    END
        <code block>
    end

This change makes accept_string_for_exception looks very similar to accept_string.
So accept_string was modified to be used for assert_same_exception and
accept_string_for_exception was removed.