Make sure that fields filled before tracker change are preserved.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11410 e93f8b46-1217-0410-a6f0-8f06a7374b81
Adds closed_on to issues API responses (#824).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11411 e93f8b46-1217-0410-a6f0-8f06a7374b81