When deleting a subtask priority of the parent task does not become changeable again (Bug #497)

I would like to patch this issue myself. (Although I am new to chiliproject/redmine codebase, I would like to contribute.) I have spent considerable amount of time looking into this issue in the Redmine/Chiliproject code base and I have come to the conclusion that this is actually a bug in Awesome Nested Set. I may be wrong here. So please guide me on how to go about fixing this bug. Here is the sample rails console session as to why I think this is a bug in Awesome Nested Set.

I am just pasting the migration from my irb-history and then
the actual console session:

class CreateCategories < ActiveRecord::Migration
  def self.up
    create_table :categories do |t|
      t.string :name
      t.integer :parent_id
      t.integer :lft
      t.integer :rgt
    end
  def self.down
    drop_table :categories
  end
end

and then CreateCategories.up

ruby-1.8.7-head >> class Category < ActiveRecord::Base
ruby-1.8.7-head?..     acts_as_nested_set
ruby-1.8.7-head?..   end
→ [
    [0] "before_move",
    [1] "after_move" 
]
ruby-1.8.7-head >> p = Category.new()
→ #<Category:0x7f6874fde128> {
           :id => nil,
         :name => nil,
    :parent_id => nil,
          :lft => nil,
          :rgt => nil
}
ruby-1.8.7-head >> p.save
  SQL (0.4ms)   SELECT max("categories".rgt) AS max_rgt FROM "categories" 
  Category Create (12.0ms)   INSERT INTO "categories" ("name", "lft", "parent_id", "rgt") VALUES(NULL, 13, NULL, 14)
→ true
ruby-1.8.7-head >> c = Category.new()
→ #<Category:0x7f6874fbf2a0> {
           :id => nil,
         :name => nil,
    :parent_id => nil,
          :lft => nil,
          :rgt => nil
}
ruby-1.8.7-head >> c.save
  SQL (0.4ms)   SELECT max("categories".rgt) AS max_rgt FROM "categories" 
  Category Create (0.3ms)   INSERT INTO "categories" ("name", "lft", "parent_id", "rgt") VALUES(NULL, 15, NULL, 16)
→ true
ruby-1.8.7-head >> p.leaf?
→ true
ruby-1.8.7-head >> c.leaf?
→ true
ruby-1.8.7-head >> c.move_to_child_of(p)
  Category Load (0.4ms)   SELECT "lft", "rgt", "parent_id" FROM "categories" WHERE ("categories"."id" = 9) 
  Category Load (0.3ms)   SELECT "lft", "rgt", "parent_id" FROM "categories" WHERE ("categories"."id" = 10) 
  Category Update (7.5ms)   UPDATE "categories" SET "lft" = CASE WHEN "lft" BETWEEN 14 AND 14 THEN "lft" + 16 - 14 WHEN "lft" BETWEEN 15 AND 16 THEN "lft" + 14 - 15 ELSE "lft" END, "rgt" = CASE WHEN "rgt" BETWEEN 14 AND 14 THEN "rgt" + 16 - 14 WHEN "rgt" BETWEEN 15 AND 16 THEN "rgt" + 14 - 15 ELSE "rgt" END, "parent_id" = CASE WHEN id = 10 THEN 9 ELSE "parent_id" END 
  Category Load (0.3ms)   SELECT "lft", "rgt", "parent_id" FROM "categories" WHERE ("categories"."id" = 9) 
  Category Load (0.2ms)   SELECT "lft", "rgt", "parent_id" FROM "categories" WHERE ("categories"."id" = 10) 
→ []
ruby-1.8.7-head >> p.save
→ true
ruby-1.8.7-head >> c.save
→ true
ruby-1.8.7-head >> p.leaf?
→ false
ruby-1.8.7-head >> c.leaf?
→ true
ruby-1.8.7-head >> p.reload
  Category Load (0.4ms)   SELECT * FROM "categories" WHERE ("categories"."id" = 9) 
→ #<Category:0x7f6874fde128> {
           :id => 9,
         :name => nil,
    :parent_id => nil,
          :lft => 13,
          :rgt => 16
}
ruby-1.8.7-head >> c.reload
  Category Load (0.5ms)   SELECT * FROM "categories" WHERE ("categories"."id" = 10) 
→ #<Category:0x7f6874fbf2a0> {
           :id => 10,
         :name => nil,
    :parent_id => 9,
          :lft => 14,
          :rgt => 15
}
ruby-1.8.7-head >> p.leaf?
→ false
ruby-1.8.7-head >> c.leaf?
→ true
ruby-1.8.7-head >> c.destroy
  Category Destroy (0.4ms)   DELETE FROM "categories" WHERE "id" = 10
→ #<Category:0x7f6874fbf2a0> {
           :id => 10,
         :name => nil,
    :parent_id => 9,
          :lft => 14,
          :rgt => 15
}
ruby-1.8.7-head >> c.save
→ true
ruby-1.8.7-head >> c.destroyed? 
→ true
ruby-1.8.7-head >> p.leaf?
→ false
ruby-1.8.7-head >> p.save
→ true
ruby-1.8.7-head >> p.leaf?
→ false
ruby-1.8.7-head >> p.reload
  Category Load (0.5ms)   SELECT * FROM "categories" WHERE ("categories"."id" = 9) 
→ #<Category:0x7f6874fde128> {
           :id => 9,
         :name => nil,
    :parent_id => nil,
          :lft => 13,
          :rgt => 16
}
ruby-1.8.7-head >> p.leaf?
→ false
ruby-1.8.7-head >> p.children 
  Category Load (0.3ms)   SELECT * FROM "categories" WHERE ("categories"."parent_id" = 9) ORDER BY "lft" 
→ []

the parent (p) of the child (c)returns false when the child is destroyed. And p.leaf? is the method that is used to control the priority enabled/disabled state in the Issue views. So I think this bug needs to be fixed in Awesome Nested Set. Please advise me as to how to go about this.

I've created a pull request here to fix this issue (also posted on the redmine tracker).