docs, link to HTTP and CGI header definitions. [ci skip]

This was a reaction to: https://github.com/rails/rails/pull/9700#issuecomment-34550210
Merge pull request #13919 from jcoyne/fix_jruby_encoding

Fix tidy_bytes for JRuby
Avoid using deprecated arel constants
Add missing directory slashes [ci skip]
Merge pull request #14007 from kintamanimatt/missing_directory_slashes

Add missing directory slashes [ci skip]
Add tip about using RDoc's --pipe command to quickly test the output of
a piece of documentation.

This can be useful for trying out combinations of markup and code, as
discussed in rails/rails@225bcad
docs, hide inflector comment targeting only contributors. [ci skip]

This is a follow up to: https://github.com/rails/rails/commit/07c70245a128cfe42f134be8759963dc98f1a63e

As suggested by @fxn this comment should not be visible in the API:
https://github.com/rails/rails/commit/07c70245a128cfe42f134be8759963dc98f1a63e#commitcomment-5331658
Merge pull request #14008 from udaykadaboina/jqueryUpgrade

Upgraded jquery-rails gem version
Renamed session_serializer option to cookies_serializer
Just very so slightly better test coverage
Added HybridSerializer to upgrade existing marshal cookies (wip: need tests)
Removed an old test
Tests for the HybridSerializer
Fixed minor typo in test code
Changed the tests to ensure HybridSerializer actually migrates the cookies (currently failing)
Convert FlashHash in a Hash with indifferent access
Stringify the incoming hash in FlashHash

Stringify the incoming as well to handle incoming symbol keys from
marshalled sessions
Re-write legacy (marshal) cookies on read
Remove serializer option from session_store.rb template
Fix AppGeneratorTest: serializer option was removed from session_store