MockChannel don't clear transaction when rollback

[KayWu]

MockChannel reuses transaction when rollback, which leaves wrong published messages in transaction.

// modify origin commit_or_rollback_can_be_called_multiple_times_after_a_single_select
String thirdMsg = "third message";
channel.basicPublish("", queue, null, thirdMsg.getBytes());
assertThat(channel.basicGet(queue, true)).isNull();
channel.txRollback();
assertThat(channel.basicGet(queue, true)).isNull();
                                                                                    
String fourthMsg = "fourth message";
channel.basicPublish("", queue, null, fourthMsg.getBytes());
assertThat(channel.basicGet(queue, true)).isNull();
channel.txCommit();
assertThat(channel.basicGet(queue, true).getBody()).isEqualTo(fourthMsg.getBytes());

[KayWu]

I raised a PR #134

[ledoyen]

In the future, don't bother opening an issue and a PR if you plan to propose a fix, the PR is sufficient (and very welcome, thank you again)