[FIXED JENKINS-19613] 49c731f made PROXY_HEADER static under the assumption that XStream would treat that like a missing field.
Unfortunately it threw an error instead, breaking startup; fixed by renaming the constant.
Merge branch 'master' of github.com:jenkinsci/jenkins
[JENKINS-19752] Noting.
Merge branch 'master' of https://github.com/jenkinsci/jenkins