Allow setting `request.variant` as an array - an order in which they will be
rendered.

For example:

  request.variant = [:tablet, :phone]

  respond_to do |format|
    format.html.none
    format.html.phone # this gets rendered
  end

Also added a `cookies_serializer.rb` initializer to the app template.

Stringify the incoming as well to handle incoming symbol keys from
marshalled sessions

This was a reaction to: https://github.com/rails/rails/pull/9700#issuecomment-34550210

If we set :shallow_path when shallow is called it can result in incorrect
paths if the resource is inside a namespace because namespace itself sets
the :shallow_path option to the namespace path.

We fix this by removing the :shallow_path option from shallow as that should
only be turning shallow routes on and not otherwise affecting the scope.
To do this we need to treat the :shallow option to resources differently to
other scope options and move it to before the nested block is called.

This change also has the positive side effect of making the behavior of the
:shallow option consistent with the shallow method.

Fixes #12498.

Since it's already required in the file, we don't need to use autoload
too. This commit is symmetrical change to 0b101804 for Response.

Returning `self` from within the array returned by `to_ary`
caused this. Instead, we can just substitute another object.
It provides the `each` behavior required by the rack spec.

cc @senny

Since the language in code blocks is inferred, if the code contains tt
tags, the block will be parsed as XML for instance while it is Ruby.

Previously, requiring action_view/view_paths did cause an uninitialized
constant error for ENCODING_FLAG, which is defined in action_view.

/cc @chancancode

We can just require the file rather than going through the autoload
indirection

Rename allowed options to :marshal and :json, for custom serializers
only allow the use of custom classes.

MessageEncryptor has :serializer option, where any serializer object can
be passed. This commit make it possible to set this serializer from configuration
level.

There are predefined serializers (:marshal_serializer, :json_serialzier)
and custom serializer can be passed as String, Symbol (camelized and
constantized in ActionDispatch::Session namepspace) or serializer object.

Default :json_serializer was also added to generators to provide secure
defalt.