Fixes issue with rails reserved keyword type exporting/importing services.

The attribute typein services was being ignored by Import/Export. Added type as a method call in the export, as type gets ignored invoking to_json, manually adding this as a method in import_export.yml solves the problem.

On a different note, I found assigning a title directly to CustomIssueTrackerService didn't play very well with prop_accessor:

> CustomIssueTrackerService.new(title: 'asdf')
NoMethodError: undefined method `[]=' for nil:NilClass
> CustomIssueTrackerService.new(title: nil)
NoMethodError: undefined method `[]=' for nil:NilClass

This was also causing the Import/Export to failed... So I added a custom setter that fixed the problem.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/22461

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?