Fix Formatting in AR Readme

Don't make free form text appear in code blocks [ci skip]
[ci skip] document type_cast_for_write
[ci skip] document ActionDispatch::HTTP::Headers
copy edits [ci skip]
fix formatting errors [ci skip]
[ci skip] doc Http::Headers methods
[ci skip] doc ActiveSupport::TimeWithZone#to_s

Current docs are wrong. Does not accept strftime inputs.
[ci skip] add example to AR#assign_attributes
copy edits [ci skip]
Merge branch 'master' of github.com:rails/docrails