Ensure self is returned during call to Nodes::Raw#add

This bug was introduced by #63.

lib/rambling/trie/nodes/raw.rb

@@ -12,6 +12,7 @@ class Raw < Rambling::Trie::Nodes::Node
         def add chars
           if chars.empty?
             terminal! unless root?
+            self
           else
             add_to_children_tree chars
           end

spec/lib/rambling/trie/nodes/raw_spec.rb

@@ -45,6 +45,10 @@ def assign_letter letter
         it 'does not mark itself as terminal' do
           expect(node).not_to be_terminal
         end
+
+        it 'returns the node' do
+          expect(add_word(node, '')).to eq node
+        end
       end
     end
 
@@ -143,6 +147,10 @@ def assign_letter letter
         it 'changes the node to terminal' do
           expect(node).to be_terminal
         end
+
+        it 'returns the node' do
+          expect(add_word(node, '')).to eq node
+        end
       end
 
       context 'when adding a one letter word' do