Add heartbeat model, next_heartbeat_time and update_heartbeat_time on device registration

spec/models/heartbeat_spec.rb

@@ -0,0 +1,15 @@
+require 'rails_helper'
+
+RSpec.describe Heartbeat, type: :model do
+  let(:heartbeat){create(:heartbeat)}
+  
+  it "update device  heartbeat details" do
+    device = heartbeat.device
+    expect(device.last_heartbeat_recd_time).to eq(heartbeat.created_at)
+    expect(device.heartbeats_count).to eq(1)
+  end
+
+  it "next heartbeat" do
+    expect(heartbeat.next_heartbeat_time).not_to be_nil
+  end
+end