Fix IssuesPdfHelperPatch

[martincizek]

• IssuesPdfHelperPatch catching up with Redmine's IssuesPdfHelper (changes related to Defect #12510 and Defect #33103).
• As the patch class contains partial reimplementation of the core IssuesPdfHelper, it could diverge again easily. A unit test was added to mitigate this.
• Hope that inheriting a unit test is ok-ish, as it is probably the only DRY and future-proof way to ensure integrity.
• This turned out to be the fix of both failing core Redmine tests. Fixes NoMethodError: undefined method `parent.subject' #225. Fixes Failing IssuesPdfHelperTest#test_fetch_row_values_should_round_float_values #226.
• Both existing and new redmine_tags tests still passing.
• Core Redmine tests are passing after this change.