This is probably an old issue.
As far as I can tell you can update an issue by posting to /job/
/config.xml with the config.xml int the body.
There still is a problem: when you GET the job config and then POST it back you would expect it to be a no-op =>
curl http://myjenkins/job/myjob/config.xml -o myjob-config.xml
curl -XPOST http://myjenkins/job/myjob/config.xml -d @myjob-config.xml
However this operation strips newlines, breaking shell steps:
<hudson.tasks.Shell> <command>echo oneecho twoecho three</command> </hudson.tasks.Shell>
(in fact the entire config.xml ends up on a single line).