Modified DelegatingStatement#close to a) null its delegate reference b) no-op on isClosed. JIRA: DBCP-415.

git-svn-id: http://svn.apache.org/repos/asf/commons/proper/dbcp@1588087 13f79535-47bb-0310-9956-ffa450edef68
Dropped needless method call.

git-svn-id: http://svn.apache.org/repos/asf/commons/proper/dbcp@1588090 13f79535-47bb-0310-9956-ffa450edef68
Fixed test comment.

git-svn-id: http://svn.apache.org/repos/asf/commons/proper/dbcp@1588103 13f79535-47bb-0310-9956-ffa450edef68