Don't use Rails.cache as fallback for session_store

It's NullStore in development by default so session does not work,
and indeed should not be dependent.

spec/integration/requests/default_bot_spec.rb

@@ -5,4 +5,9 @@ RSpec.describe DefaultBotController, :telegram_bot, type: :request do
     subject { -> { dispatch_command :start } }
     it { should respond_with_message 'from default' }
   end
+
+  describe '#load_session' do
+    subject { -> { dispatch_command :load_session } }
+    it { should_not raise_error }
+  end
 end

spec/integration/requests/other_bot_spec.rb

@@ -6,4 +6,9 @@ RSpec.describe OtherBotController, :telegram_bot, type: :request do
     subject { -> { dispatch_command :start } }
     it { should respond_with_message 'from other' }
   end
+
+  describe '#load_session' do
+    subject { -> { dispatch_command :load_session } }
+    it { should raise_error(/session_store is not configured/) }
+  end
 end