Improve poor security recommendation in docs

As reported in #9960, the current documentation recommends an insecure practice for
key generation from a password (a single round of SHA-256). The modified documentation
uses ActiveSupport::KeyGenerator to perform proper key stretching.
Fixes skipping object callback filters

This allows you to skip callbacks that are defined by objects, e.g. for
`ActionController`:

    skip_after_filter MySpecialFilter

Previously this didn't work due to a bug in how Rails compared callbacks
in `Callback#matches?`. When a callback is compiled, if it's an object
filter (i.e. not a method, proc, etc.), `Callback` now defines a method on
`@klass` that is derived from the class name rather than `@callback_id`.
So, when `skip_callback` tries to find the appropriate callback to
remove, `Callback` can regenerate the method name for the filter
object and return the correct value for `Callback#matches?`.
fix typos
Merge pull request #10280 from vipulnsward/fix_typos_temp

fix typos [ci skip]
Removing tests for deprecated options in active support. The
+use_full_path+ option was removed in 3b3790a4 and passing the template
handler to render was deprecated in 43d27e91.
Merge pull request #10283 from wangjohn/remove_tests_for_deprecated_options

Removing tests for deprecated options in active support.
Added tests for if condition in Active Model callbacks
Merge pull request #10284 from neerajdotname/missing-active-model-tests

Added tests for if condition in Active Model callbacks
fix wrong test description and failure message
Merge pull request #10286 from neerajdotname/fix-wrong-test-name-and-failure-message

fix wrong test description and failure message
fix README links in the main README.md
Merge pull request #10289 from maratvmk/readme

fix README links in the main README.md
extract path into variable
Merge pull request #10290 from buddhamagnet/master

Extract path into variable
Update counter cache when pushing into association

This commit fixes a regression bug in which counter_cache columns
were not being updated correctly when newly created records were
being pushed into an assocation. EG:

    # this was fine
    @post.comment.create!

    # this was fine
    @comment = Comment.first
    @post.comments << @comment

    # this would not update counters
    @post.comments << Comment.create!
extract array to a constant
Merge pull request #10293 from vipulnsward/dont_mutate_callback

extract array to a constant
Merge pull request #10096 from benofsky/fix_skipping_object_callback_filters

Fixes skipping object callback filters
Move CHANGELOG entry to top

[ci skip]
also assign nil in dirty nullable_datetime test. #10237