Fixes #18632

Use 'public_send' over the 'send' method for object's properties and public methods.

Speed up ActionController::Renderer `normalize_keys` by ~28%.

fix wording of CSS manifest instructions [ci skip]

Previously env was duplicated and then had it's keys mutated. This iterates through
the hash twice.
Using `transform_keys`, duplication and key mutation is a single iteration.

`convert_symbols` was renamed to `http_header_format`.

Disable builds AR-JDBC against master

fix regexp for validate an integer in guides [ci skip]

Remove config.assets

A shortcut to setup controller environment

Currently, bundling `activerecord-jdbc-adapter`'s master branch seems to
be broken. See jruby/activerecord-jdbc-adapter#614

This commit partially reverts #12107

use two spaces instead of one for include root in json

Add :formats to options list of render documentation.

Provided by sprockets-rails plugin

Render arbitrary templates outside of controller actions

To have an easier way to setup a controller
instance with custom environment

Add `ActionController::Metal#set_request!` to set a request
on controller instance without calling dispatch.

Keeping with our behavior elsewhere in the system, invalid input is
assumed to be `nil`.

Fixes #18629.

Add ActiveModel::Errors#codes

Replace `if exists` with `table_exists?` and drop table with `drop_table`

since 'drop table if exists' statement does not always work with some databases such as Oracle.
also Oracle drop table statement will not drop sequence objects.

Travis: Test on jruby-head only for now

Arel head does not support 1.9 anymore.

Add missing AS core extension dependency

Add missing options to Error Message Interpolation guide [CI skip]

Change 'a' to 'an' for 'attribute' word [ci skip]

This method can be used to see all of the fields on a model which have
been read. This can be useful during development mode to quickly find
out which fields need to be selected. For performance critical pages, if
you are not using all of the fields of a database, an easy performance
win is only selecting the fields which you need. By calling this method
at the end of a controller action, it's easy to determine which fields
need to be selected.

While writing this, I also noticed a place for an easy performance win
internally which I had been wanting to introduce. You cannot mutate a
field which you have not read. Therefore, we can skip the calculation of
in place changes if we have never read from the field. This can
significantly speed up methods like `#changed?` if any of the fields
have an expensive mutable type (like `serialize`)

```
Calculating -------------------------------------
 #changed? with serialized column (before)
                       391.000  i/100ms
 #changed? with serialized column (after)
                         1.514k i/100ms
-------------------------------------------------
 #changed? with serialized column (before)
                          4.243k (± 3.7%) i/s -     21.505k
 #changed? with serialized column (after)
                         16.789k (± 3.2%) i/s -     84.784k
```

To be able to return type of validator, one can now call `details`
on Errors instance:

```ruby
class User < ActiveRecord::Base
  validates :name, presence: true
end
```

```ruby
user = User.new; user.valid?; user.errors.details
=> {name: [{error: :blank}]}
```