Fix error when using `with_options` with lambda.

It was causing error when using `with_options` passing a lambda as its
last argument.

    class User < ActiveRecord::Base
      with_options dependent: :destroy do |assoc|
        assoc.has_many :profiles, -> { where(active: true) }
      end
    end

It was happening because the `option_merger` was taking the last
argument and checking if it was a Hash. This breaks the HasMany usage,
because its last argument can be a Hash or a Proc.

As the behavior described in this test:
https://github.com/rails/rails/blob/master/activesupport/test/option_merger_test.rb#L69
the method will only accept the lambda, this way it will keep the expected behavior. See 9eaa0a34
Explicitly load Kernel#y when starting a console

Previously, we relied on the IRB-detection in Psych itself. But that
doesn't work when we're running under spring: the application boots (and
thus psych is required) before we switch to console mode and load IRB.

Fixes #14587.
Merge pull request #14588 from matthewd/kernel_y_with_spring

Explicitly load Kernel#y when starting a console
Improve error message for guides:generate:kindle

With `kindlegen` missing from the PATH, the task
keeps suggesting to `gem install kindlerb`.
This change provides a more meaningful error
message for guides:generate:kindle.
Merge pull request #14590 from czak/improved_kindlerb_error_message

Improve error message for guides:generate:kindle
Merge pull request #14572 from laurocaetano/with_options_and_scope

Fix error when using `with_options` with lambda.

Conflicts:
	activerecord/CHANGELOG.md