Remove XML Serialization from core

Discussion https://github.com/JuanitoFatas/fast-ruby/pull/59#issuecomment-128513763

This includes the following classes:

- ActiveModel::Serializers::Xml
- ActiveRecord::Serialization::XmlSerializer

this is another place that we should stop directly accessing the env
hash and let the request object take care of that for us

in_batches using ids

Fix doc typo

Prevent the default block for mattr_accessor being called multiple times

Only invoke the default block for mattr_accessor once so that it does not cause issues if it is not idempotent

`in_batches` yields Relation objects if a block is given, otherwise it
returns an instance of `BatchEnumerator`. The existing `find_each` and
`find_in_batches` methods work with batches of records. The new API
allows working with relation batches as well.

Examples:

    Person.in_batches.each_record(&:party_all_night!)
    Person.in_batches.update_all(awesome: true)
    Person.in_batches.delete_all
    Person.in_batches.map do |relation|
      relation.delete_all
      sleep 10 # Throttles the delete queries
    end

[ci skip] Remove `identity.active_record`

This is removed by this commit
cf: https://github.com/rails/rails/pull/5261

Speed up code and avoid unnecessary MatchData objects

do not add common ports to HTTP_HOST

Added bin/update script to update application automatically

Fix documentation on ActionDispatch::Request

use system!

fix changelog

use bundle check first and use rake

use system instead system! for bundle check

 - webservers do not do it
 - it makes redirect urls ugly when request.host is used for redirection

again, we want to hide the contents of `env` from the implementation.
Allocate a request object to access the contents of env, but save
allocations due to string literal allocations when accessing the env
hash.

hide the env key in the request object so that other code doesn't need
to know.

ExceptionWrapper only cares about the backtrace cleaner, so lets just
pass the cleaner to the wrapper.  It does not need to know that env
exists or what key the backtrace cleaner is stored in

Implement `serve` on the middleware.  Nothing can be placed between the
instance of FileHandler and Static because Static instantiates an
instance of FileHandler.  IOW there is no reason to implement the `call`
API in this case.

Use memoization for collection associations ids reader

Fix STATS_DIRECTORIES warning by only loading statistics.rake once

When running rake stats from inside an engine,
the engine's Rakefile attempts to reload
statistics.rake after the test app loads it, which
results in STATS_DIRECTORIES being redefined and
an annoying warning. This patch skips loading
statistics.rake from tasks.rb if rake's current
scope isn't empty, i.e. if we are running from
inside an engine and not the test app dir or a
normal app.

Fixes #20510.

set the correct path to `ActionDispatch::IntegrationTest.fixture_path`

Remove duplicated `Array#to_param`

Fix Encoding::UndefinedConversionError with multibyte UTF-8 filename containing "%" character

Fix test session fetch

Better docs for ActiveRecord::Migration#table_name_options

Small fix [ci skip]