Update device create API

- Update the existing device if a device with existing unique_id is
   present

spec/controllers/devices_controller_spec.rb

@@ -4,10 +4,27 @@ RSpec.describe DevicesController, type: :controller do
 
   describe "POST #create" do
     it "creates a device" do
-      expect {
+      expect do
         post :create, device: attributes_for(:device), format: :json
-      }.to change{ Device.count }.by(1)
-      #expect(response).to have_http_status(:success)
+      end.to change{ Device.count }.by(1)
+    end
+
+    describe "when device with existing unique id comes" do
+      before(:example) do
+        device = create(:device)
+        @device_params = { model: "New Device", unique_id: device.unique_id }
+      end
+
+      it "does not create a new device" do
+        expect do
+          post :create, device: @device_params, format: :json
+        end.to change { Device.count }.by(0)
+      end
+
+      it "updated the existing device with new params" do
+        post :create, device: @device_params, format: :json
+        expect(Device.last.model).to eq("New Device")
+      end
     end
   end