Make `Hanami::Action::{Request,Response}` `#session` and `#flash` to raise `Hanami::Controller::MissingSessionError` when session is not configured

[jodosha]

With https://github.com/hanami/controller/pull/250 (v1.2.0) we introduced a new exception (Hanami::Controller::MissingSessionError) which is raised when a dev tries to access Action#session and Action#flash when the HTTP session isn't configured.

In 2.0 (unstable branch), the access to session and flash will be moved from Hanami::Action to Hanami::Action::Request and Response. These two objects need to support the old behavior of raising a Hanami::Controller::MissingSessionError.

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This change IS for unstable branch.

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Ref https://github.com/hanami/controller/pull/250

[GustavoCaso]

Sorry, that was the wrong click 😄

[timriley]

Fixed in #379.