Project#all_issue_custom_fields now returns a scope.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11916 e93f8b46-1217-0410-a6f0-8f06a7374b81
Query#add_custom_fields_filters now takes a custom fields scope.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11917 e93f8b46-1217-0410-a6f0-8f06a7374b81