fixture setup does not rely on `AR::Base.configurations`.

As you can also configure your database connection using `ENV["DATABASE_URL"]`,
the fixture setup can't reply on the `.configurations` Hash.

As the fixtures are only loaded when ActiveRecord is actually used
(`rails/test_help.rb`) it should be safe to drop the check for an existing configuration.

'json' gem is no more required under JRuby

regression test + mysql2 adapter raises correct error if conn is closed.

fix typos

Fix wrong method name

We need to check exception name to be nil

We need to return Regexp.escape(camel_cased_word)
if the split is blank.

[ci skip]

HashWithIndifferentAccess#select working as intended

Refactored ActiveRecord `first` method to get rid of duplication.

Added an example of the query without except before the example with except

Fix #10932. Treat "" and "::" as invalid on constantize

`initialize_on_precompile` is not used anymore.

recreate the models without mucking with internal caches of the relation
objects, then the test fails.

For example:

class Man < ActiveRecord::Base
  has_many :interests
end

class Interest < ActiveRecord::Base
  belongs_to :man
end

Then do this test:

def test_validate_presence_of_parent_fails_without_inverse_of
  repair_validations(Interest) do
    Interest.validates_presence_of(:man)
    assert_no_difference ['Man.count', 'Interest.count'] do
      man = Man.create(:name => 'John',
                       :interests_attributes => [{:topic=>'Cars'}, {:topic=>'Sports'}])
      assert_not_predicate man.errors[:"interests.man"], :empty?
    end
  end
end

The test will fail.  This is a bad test, so I am removing it.

We are setting this header to chrome=1 for Chrome Frame and this will be
retired soon. Check http://blog.chromium.org/2013/06/retiring-chrome-frame.html for
details

Conflicts:
	guides/source/upgrading_ruby_on_rails.md

This reverts commit 6df9c595.

This reverts commit 91a1cf70.

Reason: code changes disallowed in docrails, even if they are
guides generation related.