Add happy path test cases to Exchange folder encoding fix

spec/lib/import/exchange/folder_spec.rb

@@ -10,6 +10,29 @@ RSpec.describe Import::Exchange::Folder do
     let(:child_folder)   { double('EWS Folder') }
     let(:exception_case) { double('EWS Folder') }
 
+    context 'when server returns valid UTF-8' do
+      before do
+        allow(root_folder).to receive(:display_name).and_return('Root')
+        allow(root_folder).to receive(:parent_folder_id).and_return(nil)
+
+        allow(child_folder).to receive(:display_name).and_return('Leaf')
+        allow(child_folder).to receive(:parent_folder_id).and_return(1)
+
+        allow(exception_case).to receive(:display_name).and_return('Error-Raising Leaf')
+        allow(exception_case).to receive(:parent_folder_id).and_raise(Viewpoint::EWS::EwsError)
+
+        allow(subject).to receive(:find).with(any_args).and_return(root_folder)
+        allow(subject).to receive(:find).with(nil).and_return(nil)
+      end
+
+      it 'returns a valid UTF-8 string' do
+        expect(subject.display_path(root_folder)).to eq('Root')
+        expect(subject.display_path(child_folder)).to eq('Root -> Leaf')
+        expect(subject.display_path(exception_case)).to eq('Error-Raising Leaf')
+      end
+    end
+
+    context 'when server returns invalid UTF-8' do
       before do
         allow(root_folder).to receive(:display_name).and_return('你好'.b)
         allow(root_folder).to receive(:parent_folder_id).and_return(nil)
@@ -31,3 +54,4 @@ RSpec.describe Import::Exchange::Folder do
       end
     end
   end
+end