Fix GttMapLayer position default (0 to nil)

[sanak]

Fixes #274.

Changes proposed in this pull request:

• Fix GttMapLayer position default (0 to nil)

Explanation:

1. acts_as_positioned is defined in the following Redmine core side, and before_save - set_default_position assumes that position default is nil.
   • https://github.com/redmine/redmine/blob/5.1-stable/lib/redmine/acts/positioned.rb#L35-L44

     def acts_as_positioned(options = {})
       class_attribute :positioned_options
       self.positioned_options = {:scope => Array(options[:scope])}
     
       send :include, Redmine::Acts::Positioned::InstanceMethods
     
       before_save :set_default_position
       after_save :update_position
       after_destroy :remove_position
     end

   • https://github.com/redmine/redmine/blob/5.1-stable/lib/redmine/acts/positioned.rb#L72-L76

     def set_default_position
       if position.nil?
         self.position = position_scope.maximum(:position).to_i + (new_record? ? 1 : 0)
       end
     end

2. But current migration specifies position default with 0.
   • https://github.com/gtt-project/redmine_gtt/blob/next/db/migrate/20171010022000_add_position_to_gtt_tile_sources.rb

     class AddPositionToGttTileSources < ActiveRecord::Migration[5.2]
       def change
         add_column :gtt_tile_sources, :position, :integer, default: 0
       end
     end

3. There was referable past migration in Redmine core side
   • https://github.com/redmine/redmine/blob/5.1-stable/db/migrate/20160416072926_remove_position_defaults.rb

     class RemovePositionDefaults < ActiveRecord::Migration[4.2]
       def up
         [Board, CustomField, Enumeration, IssueStatus, Role, Tracker].each do |klass|
           change_column klass.table_name, :position, :integer, :default => nil
         end
       end
     
       def down
         [Board, CustomField, Enumeration, IssueStatus, Role, Tracker].each do |klass|
           change_column klass.table_name, :position, :integer, :default => 1
         end
       end
     end

4. So, I applied same from the following migrate command (link) and above except default 1 to 0.

   bundle exec rails generate redmine_plugin_migration redmine_gtt remove_gtt_map_layer_position_default

I confirmed that this changes fix the issue in #274.

@gtt-project/maintainer

[sanak]

Well, I will consider to add some tests which fail without this migration.