Make session creation lazier

Don't create a session just because someone attempted to read data from
one.  I.e. just because someone checks C< session("logged_in") >> don't
create a new, empty session.

Addresses GH Issue #155

lib/Dancer2/Core/App.pm

@@ -150,6 +150,12 @@ sub engine {
 sub session {
     my ($self, $key, $value) = @_;
 
+    # shortcut reads if no session exists, so we don't
+    # instantiate sessions for no reason
+    if ( @_ == 2 ) {
+        return unless $self->context->has_session;
+    }
+
     my $session = $self->context->session;
     croak "No session available, a session engine needs to be set"
       if !defined $session;

t/session_lifecycle.t

@@ -51,21 +51,20 @@ foreach my $engine (@engines) {
             ok !$cookie, "no cookie set"
               or diag explain $cookie;
 
-            # empty session created if session read attempted
+            # no empty session created if session read attempted
             $res = $ua->get("http://127.0.0.1:$port/read_session");
             ok $res->is_success, "/read_session";
             $cookie = extract_cookie($res);
-            ok $cookie, "session cookie set"
+            ok !$cookie, "no cookie set"
               or diag explain $cookie;
-            my $sid1 = $cookie->{"dancer.session"};
-            like $res->content, qr/name=''/, "empty session";
 
             # set value into session
             $res = $ua->get("http://127.0.0.1:$port/set_session/larry");
             ok $res->is_success, "/set_session/larry";
             $cookie = extract_cookie($res);
             ok $cookie, "session cookie set"
               or diag explain $cookie;
+            my $sid1 = $cookie->{"dancer.session"};
 
             # read value back
             $res = $ua->get("http://127.0.0.1:$port/read_session");