Session::configure overrides setHandler settings

[lexaurin]

Imagine this scenario:

$session->setOptions(array('save_handler' => 'redis');
$session->setHandler(new RedisStorageExtension);
$session->start(); //overrides my custom handler with native 'redis'

Nette/Session overrides custom handler on Session::start();

In my situation I cannot leave save_handler with default files value because my handler extends Redis handler, not the default one.
Setting handler after session start could help but session start is lazy and we don't know when it happens.

Edit: Documentation about session handler extensions http://php.net/manual/en/class.sessionhandler.php

[lexaurin]

@dg What about this one?

Without this change it's not possible to use SessionHandler class extension with Nette\Http\Session.

I know, test case with dependency on Redis extension is bad, but I don't know how to avoid it. I need to test switching from default handler files to something else but what "something else" should I use in test.. ?

Latest version based on master 0d8d21f