If a counter_cache exists, use it for #empty?
Add ability to extend `rails server` command options parser

With this change it will be possible to add additional options to the `option_parser` like this:

    require 'rails/commands/server'
    module Rails
      class Server < ::Rack::Server
        class Options
          def option_parser_with_open(options)
            parser = option_parser_without_open options
            parser.on('-o', '--open', 'Open in default browser') { options[:open] = true }
            parser
          end
          alias_method_chain :option_parser, :open
        end

        def start_with_open
          start_without_open do
            `open http://localhost:3000` if options[:open]
          end
        end
        alias_method_chain :start, :open
      end
    end
[CI SKIP] Doc fix.
Log digest as :debug instead of :info
Merge pull request #16174 from tgxworld/doc_fix

[ci skip] Fix grammar of eager loading for ActiveRecord::Associations
Return an absolute instead of relative path from an asset url in the case of the `asset_host` proc returning nil
Merge pull request #16161 from jpawlyn/master

Fix empty host for an asset url when asset_host proc returns nil
Merge pull request #16175 from skorks/log_digestor_as_debug

Log digest as :debug instead of :info
Merge pull request #16142 from andreychernih/extend-server-options

Add ability to extend `rails server` command options parser
Merge pull request #15266 from dv/use_counter_cache_for_empty_call

If a counter_cache exists, use it for #empty?
Add CHANGELOG entry for #15266
Fix version detection for RENAME INDEX support. Fixes #15931.
Revert "Revert "Merge pull request #16059 from jenncoop/json-serialized-attr""

This reverts commit 6f3c64eeb1dc8288dae49f114aaf619adc7dcb7f.

Conflicts:
	activerecord/CHANGELOG.md
Predicate methods don't need to return `true` / `false`
Merge pull request #16162 from chancancode/fix_json_coder

Fixed JSON coder when loading NULL from DB
Fixed SQL syntax for postgresql
Merge branch 'jbrowning-fix_mariadb_rename_index'

Closes #15936
Document the change in `nil` handling for serialized attributes

Also updated the test case to reflect that
Merge branch 'master' of git://github.com/rails/rails

Conflicts:
	activerecord/CHANGELOG.md
In eager loading association code sample, modified to fetch last comment