1. Create a job which polls with http://localhost/foo.txt and monitors a change of the content.
2. 1st poll result is "No changes". (expected)
3. Edit foo.txt on localhost.
4. 2nd poll result is "There are changes". (expected)
5. 3rd poll result is "There are changes". (NOT expected)
6. Go to the job configuration page, and click "save". There is no need to edit the configuration.
7. 4th poll result is "No changes". (expected)

If ClientResponse.getEntity is called multiple times,
its return value will be an empty string.
A workaround would be https://github.com/ohtake/urltrigger-plugin/commit/f9c3cbde37a7f46e7fa6e14540fa6d0a96312c29.
It is not a neat commit, but it works for me.