Update project_decorator.rb

Ported redmine Rev12131 to this plugin.
http://www.redmine.org/projects/redmine/repository/revisions/12131/diff/trunk/app/models/issue.rb

I've detected, that the calculation will break if the estimated_hours were entered, but be just 0. This will cause some weird side effects. This should fix that.
Merge pull request #25 from CodeInsider/patch-3

Fix calculation break if the estimated_hours were entered, but be just 0.