Patch for single file inclusion through spring's classpath resource accessor

Just avoid having a file not found for _path/to/mychangelog.groovy/mychangelog.groovy_ when including a single changeset file this way
```groovy
include(path: 'path/to/mychangelog.groovy', relativeToChangelog: false)
```
(proposed refinement for [pull request 31] (https://github.com/tlberglund/groovy-liquibase/pull/31), see https://github.com/tlberglund/groovy-liquibase/issues/28 )