Add :ttl configuration option (#84)

**Why**: The :expire_after config option controls both the Redis
session expiration and the expiration of the Rails session cookie.
Adding a new config option :ttl allows those controls to be
managed separately.

The most common case would be if you wanted the Rails cookie
to expire when the browser is closed, as it would if :expire_after
were set to `nil`.

README.md

@@ -36,7 +36,8 @@ In your Rails app, throw in an initializer with the following contents:
 Rails.application.config.session_store :redis_session_store, {
   key: 'your_session_key',
   redis: {
-    expire_after: 120.minutes,
+    expire_after: 120.minutes,  # cookie expiration
+    ttl: 120.minutes,           # Redis expiration, defaults to 'expire_after'
     key_prefix: 'myapp:session:',
     url: 'redis://localhost:6379/0',
   }

lib/redis-session-store.rb

@@ -116,7 +116,7 @@ def decode(data)
   end
 
   def set_session(env, sid, session_data, options = nil)
-    expiry = (options || env.fetch(ENV_SESSION_OPTIONS_KEY))[:expire_after]
+    expiry = get_expiry(env, options)
     if expiry
       redis.setex(prefixed(sid), expiry, encode(session_data))
     else
@@ -129,6 +129,11 @@ def set_session(env, sid, session_data, options = nil)
   end
   alias write_session set_session
 
+  def get_expiry(env, options)
+    session_storage_options = options || env.fetch(ENV_SESSION_OPTIONS_KEY, {})
+    session_storage_options[:ttl] || session_storage_options[:expire_after]
+  end
+
   def encode(session_data)
     serializer.dump(session_data)
   end

spec/redis_session_store_spec.rb

@@ -59,6 +59,29 @@
     end
   end
 
+  describe 'when configured with both :ttl and :expire_after' do
+    let(:ttl_seconds) { 60 * 120 }
+    let :options do
+      {
+        key: random_string,
+        secret: random_string,
+        redis: {
+          host: 'hosty.local',
+          port: 16_379,
+          db: 2,
+          key_prefix: 'myapp:session:',
+          ttl: ttl_seconds,
+          expire_after: nil
+        }
+      }
+    end
+
+    it 'assigns the :ttl option to @default_options' do
+      expect(default_options[:ttl]).to eq(ttl_seconds)
+      expect(default_options[:expire_after]).to be_nil
+    end
+  end
+
   describe 'when initializing with top-level redis options' do
     let :options do
       {