fix #895 by clearing the repeat pattern from todos before the pattern is deleted

cherry-picked from 1.7branch

app/controllers/contexts_controller.rb

@@ -121,6 +121,9 @@ class ContextsController < ApplicationController
   # actions, you'll get a warning dialogue. If you choose to go ahead, any
   # actions in the context will also be deleted.
   def destroy
+    # make sure the deleted recurring patterns are removed from associated todos
+    @context.recurring_todos.each { |rt| rt.clear_todos_association } unless @context.recurring_todos.nil?
+    
     @context.destroy
     respond_to do |format|
       format.js { @down_count = current_user.contexts.size }

app/models/recurring_todo.rb

@@ -717,6 +717,15 @@ class RecurringTodo < ActiveRecord::Base
     self.save
   end
 
+  def clear_todos_association
+    unless todos.nil?
+      self.todos.each do |t|
+        t.recurring_todo = nil
+        t.save
+      end
+    end
+  end
+  
   def inc_occurences
     self.occurences_count += 1
     self.save