(bankruptcy debtor’s right to voluntarily dismiss a Chapter 13 case under 11 U.S.C. §1307(b) is not absolute, but is qualified by an implied exception for bad-faith conduct or abuse of the bankruptcy process; bankruptcy court did not err in finding bad faith conduct in present case, and even though court failed to provide debtor with adequate notice and hearing before converting Chapter 13 case to Chapter 7, debtor failed to show prejudice).