Extract behavior from Todo.from_rich_message

`Todo.from_rich_message` was partially tested from the MessageGateway,
but it is mostly untested and does not belong in Todo due to it's unique
nature. Extract it entirely from `Todo` into it's own dedicated object
for unit testing.

test/models/todo_from_rich_message_test.rb

@@ -0,0 +1,21 @@
+require File.expand_path(File.dirname(__FILE__) + '/../test_helper')
+
+class TodoFromRichMessageTest < ActiveSupport::TestCase
+
+  def setup
+    @completed = Todo.find(8)
+  end
+
+  def test_from_rich_message_adds_to_default_context
+    user = @completed.user
+    default_context_id = @completed.context_id
+    builder = TodoFromRichMessage.new(user, default_context_id, "new todo", "notes")
+    new_todo = builder.construct
+
+    assert_not_nil new_todo
+    assert_equal "new todo", new_todo.description
+    assert_equal "notes", new_todo.notes
+    assert_equal default_context_id, new_todo.context_id
+  end
+
+end

test/models/todo_test.rb

@@ -477,15 +477,4 @@ class TodoTest < ActiveSupport::TestCase
     assert_equal "<p><strong>test</strong></p>", todo.rendered_notes
   end
 
-  def test_from_rich_message_adds_to_default_context
-    user = @completed.user
-    default_context_id = @completed.context_id
-    new_todo = Todo::from_rich_message(user, default_context_id, "new todo", "notes")
-
-    assert_not_nil new_todo
-    assert_equal "new todo", new_todo.description
-    assert_equal "notes", new_todo.notes
-    assert_equal default_context_id, new_todo.context_id
-  end
-  
 end