Update PrefixedTubePheanstalkProxy.php

adds the ability to self-define tube prefix (`getTubePrefix` can do some operations which can alter tube's name) without the need for it to be injected to the class from elsewhere
Update PrefixedTubePheanstalkProxyTest.php
Merge pull request #1 from prgTW/patch-1

Using getTubePrefix() interally.