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

The attribute `type`in 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`:

```ruby
> 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

See merge request !6499