URLResource#isExists returns true for non-existent http and ftp URLs, based on patch by Anthony Wat, PR 51110

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@1577796 13f79535-47bb-0310-9956-ffa450edef68
some of our tests require JUnit4

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@1577797 13f79535-47bb-0310-9956-ffa450edef68