safe() also replaces '%' with '_'

`TestObject#safe` replaces `%` with `_` in addition to the other characters it replaces.

The [Cucumber Test Result Plugin](https://wiki.jenkins-ci.org/display/JENKINS/Cucumber+Test+Result+Plugin) converts Cucumber feature and scenario names into hyperlinks for reviewing test results. Feature and scenario names can have any characters into them, including non-URL-safe ones.

This patch fixes the problem for names containing `%`.