DEV: correct implementation of expiry api

Previously we were always hard-coding expiry, this allows the secure session
to correctly handle custom expiry times

Also adds a ttl method for looking up time to live

spec/components/secure_session_spec.rb

@@ -14,4 +14,16 @@ describe SecureSession do
     s["hello"] = nil
     expect(s["hello"]).to eq(nil)
   end
+
+  it "can override expiry" do
+    s = SecureSession.new("abc")
+    key = SecureRandom.hex
+
+    s.set(key, "test2", expires: 5.minutes)
+    expect(s.ttl(key)).to be_within(1.second).of (5.minutes)
+
+    key = SecureRandom.hex
+    s.set(key, "test2")
+    expect(s.ttl(key)).to be_within(1.second).of (SecureSession.expiry)
+  end
 end