Make #session raise error when store is not configured

Add more explanation on sessions in readme

spec/telegram/bot/updates_controller/session_spec.rb

@@ -54,4 +54,45 @@ RSpec.describe Telegram::Bot::UpdatesController::Session do
       end
     end
   end
+
+  describe '.build_session' do
+    subject { controller_class.build_session(key, *args) }
+    let(:key) {}
+    let(:args) { [] }
+    it { expect { subject }.to raise_error(/session_store is not configured/) }
+
+    shared_examples 'NullSessionHash when key is not present' do |store_proc|
+      it { should be_instance_of(described_class::NullSessionHash) }
+
+      context 'and key is present' do
+        let(:key) { :test_key }
+        it 'is valid SessionHash' do
+          expect(subject).to be_instance_of(described_class::SessionHash)
+          expect(subject.id).to eq key
+          expect(subject.instance_variable_get(:@store)).to be(instance_exec(&store_proc))
+        end
+      end
+    end
+
+    context 'when store configured' do
+      before { controller_class.session_store = nil }
+      include_examples 'NullSessionHash when key is not present',
+        -> { controller_class.session_store }
+    end
+
+    context 'when store is given' do
+      let(:args) { [double(:store)] }
+      include_examples 'NullSessionHash when key is not present', -> { args[0] }
+    end
+  end
+
+  describe '.session_store=' do
+    subject { ->(val) { controller_class.session_store = val } }
+    it 'casts to AS::Cache' do
+      expect { subject[:null_store] }.to change(controller_class, :session_store).
+        to(instance_of(ActiveSupport::Cache::NullStore))
+      expect { subject[nil] }.to change(controller_class, :session_store).
+        to(instance_of(ActiveSupport::Cache::MemoryStore))
+    end
+  end
 end