Wrong done_ratio calculation for parent with subtask having estimated_hours=0 ().

Patch by Daniel Felix.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12131 e93f8b46-1217-0410-a6f0-8f06a7374b81
Test for #14798.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12132 e93f8b46-1217-0410-a6f0-8f06a7374b81