In tests, change child_count cache column to default to 0

Before
======

New records had the children counter cache value of nil.

On mysql, update_counter_cache sets nodes with no children to nil.

After
=====

New records have the children counter cache set to 0

On mysql, update_counter_cache now sets to 0 (was nil)
new records set the nodes with no children to have a count of 0.

Docs
====

Looks like there is no documentation for the child counter cache.
Need to add to the README\

lib/ancestry/class_methods.rb

@@ -241,7 +241,7 @@ def rebuild_counter_cache!
             JOIN #{table_name} children ON children.#{ancestry_column} = (#{child_ancestry_sql})
             GROUP BY #{table_name}.#{primary_key}
           ) src USING(#{primary_key})
-          SET dest.#{counter_cache_column} = src.child_count
+          SET dest.#{counter_cache_column} = COALESCE(src.child_count, 0)
         }
       else
         update_all %{

test/concerns/sti_support_test.rb

@@ -79,10 +79,10 @@ def test_sti_support_for_counter_cache
       node5 = Subclass3.create! :parent => node4
 
       assert_equal 2, node1.reload.children_count
-      assert_nil node2.reload.children_count
+      assert_equal 0, node2.reload.children_count
       assert_equal 1, node3.reload.children_count
       assert_equal 1, node4.reload.children_count
-      assert_nil node5.reload.children_count
+      assert_equal 0, node5.reload.children_count
 
       Object.send :remove_const, 'Subclass3'
       Object.send :remove_const, 'Subclass4'

test/environment.rb

@@ -129,7 +129,7 @@ def self.with_model options = {}
       table.integer options[:cache_depth] == true ? :ancestry_depth : options[:cache_depth] if options[:cache_depth]
       if options[:counter_cache]
         counter_cache_column = options[:counter_cache] == true ? :children_count : options[:counter_cache]
-        table.integer counter_cache_column
+        table.integer counter_cache_column, default: 0, null: false
       end
 
       extra_columns.each do |name, type|