Change ActionController::Parameters#require behavior when value is empty
When the value for the required key is empty an ActionController::ParameterMissing is raised which gets caught by ActionController::Base and turned into a 400 Bad Request reply with a message in the body saying the key is missing, which is misleading.

With these changes, ActionController::EmptyParameter will be raised which ActionController::Base will catch and turn into a 400 Bad Request reply with a message in the body saying the key value is empty.
Fix doc in Postgres database creation
allow to test AR master against AR-JDBC master (with `ENV['AR_JDBC']`)
`export AR_JDBC=true` on travis-ci as a *before_install* (but on jruby only)
Moved all JSON core extensions into core_ext/object/json

TL;DR The primary driver is to remove autoload surprise.

This is related to #12106. (The root cause for that ticket is that
json/add defines Regexp#to_json among others, but here I'll reproduce
the problem without json/add.)

Before:

   >> require 'active_support/core_ext/to_json'
   => true
   >> //.as_json
   NoMethodError: undefined method `as_json' for //:Regexp
     from (irb):3
     from /Users/godfrey/.rvm/rubies/ruby-2.0.0-p195/bin/irb:16:in `<main>'
   >> //.to_json
   => "\"(?-mix:)\""
   >> //.as_json
   => "(?-mix:)"

After:

   >> require 'active_support/core_ext/to_json'
   => true
   >> //.as_json
   => "(?-mix:)"

This is because ActiveSupport::JSON is autoloaded the first time
Object#to_json is called, which causes additional core extentions
(previously defined in active_support/json/encoding.rb) to be loaded.

When someone require 'active_support/core_ext', the expectation is
that it would add certain methods to the core classes NOW. The
previous behaviour causes additional methods to be loaded the first
time you call `to_json`, which could cause nasty surprises and other
unplesant side-effects.

This change moves all core extensions in to core_ext/json. AS::JSON is
still autoloaded on first #to_json call, but since it nolonger
include the core extensions, it should address the aforementioned bug.

*Requiring core_ext/object/to_json now causes a deprecation warnning*
Add failing test for preloading with a polymorphic association and using the existential predicate
Fix broken delete_all test, which will now be failing since #delete_all is broken
Merge pull request #10664 from aderyabin/docpatch

Fix doc in Postgres database creation [ci skip]
Revert "add active model basics wip to guides/documents.yaml"

Reason: WIP guides are not in the index because they are partial
work pushed to the repo. Either there is someone working on them
or else the work was interrupted.

Guides are added to the index page when they are finished, the
author considers the draft to be complete, someone reviews them,
and overall they are considered to be good for users to read.

This reverts commit eefc03a8069bddfe31751c4bd4fb1804e7f33c2c.
Minor doc fix of ActiveModel::Naming.

- qoute example line's result when it kind of String
- right ("singular_route_key") method in example
Update testing.rake with more accurate description of test task.

According to #5ecd12 change.

[ci skip]
Merge pull request #12688 from simi/patch-1

Update testing.rake with more accurate description of test task.
Clarify that visiting http://localhost:3000 should be done in a browser.
Merge pull request #12690 from durrantm/minor_README_change

Clarify that visiting http://localhost:3000 should be done in a browser. [ci skip]
Convert CDATA input to string before gsub'ing

Rails 3.2 API allowed arbitrary input for cdata_section;
this change re-introduces the old behaviour.
Merge pull request #12693 from carpodaster/convert-cdata-input-to-string

Convert CDATA input to string before gsub'ing
add the fetch method to sessions
Merge pull request #12692 from dmathieu/session_fetch

Add session#fetch
short arrays in inspect
Merge pull request #12653 from releu/short_arrays_in_inspect

Short arrays in record.inspect