Fix formats on xhr requests when HTTP_ACCEPT is empty string

Fixed typo in active record's changelog

Fix ActionDispatch::Request#formats on xhr requests when HTTP_ACCEPT
header is empty string. About issue #7774, same fix as in commit bebb02fd
but for xhr requests.

add prompt to 422 page to check logs

Introduce relation #unscope

In example fix sql that is generated from scoping 

relations. Specific where values can be unscoped, and the unscope method
still works when relations are merged or combined.

Since the Rack::Lock still exists in development,
let's provide a way to disable it explicitly.

Rather than trying to use gsub to remove the optional route segments,
which will fail with nested optional segments, use a custom visitor
class that returns a empty string for group nodes.

Closes #9524

integrate the strong params README into the AC guide.

The current ActionController guide does not mention strong parameters
at all. I integrated the README into the guide to explain the API.

I also included a section to illustrate that the API does not solve
all possible whitelisting scenarios.

The origin was #9454.

New 404, 422, 500 pages, they are more stylish and bring ruby essence…

Fix PostgreSQL TIMESTAMP WITH TIME ZONE to return ActiveSupport::Time

deal with `#append` and `#prepend` on association collections

Closes #7364.

Collection associations behave similar to Arrays. However there is no
way to prepend records. And to append one should use `<<`. Before this
patch `#append` and `#prepend` did not add the record to the loaded
association.

`#append` now behaves like `<<` and `#prepend` is not defined.

moving changelog note to unreleased section

New beauty pages(404, 422, 500)

In an AR model a timestamptz attribute would return a ruby string and AR
tests did not check for any type casting.  Previous tests would pass
only because an assert_equal was being used on a Time.utc object, which
will parse the right side of the eq to a valid Time instance for
comparision.

switch to test instance of Time instead of ActiveSupport::TimeWithZone

Support creating a table migration generator

Sometimes you want to create a table without an associated model and
test, which is also not a join table. With this commit, you can now
do that.

Example:

    rails g migration create_posts title:string
or
    rails g migration CreatePosts title:string

This commit also moves the template the model generator uses for the
migration to the migration templates folder, as it seems a more
sensible place for it now that it is shared code.

Sometimes you want to create a table without an associated model and
test, which is also not a join table. With this commit, you can now
do that.

Example:

    rails g migration create_posts title:string
or
    rails g migration CreatePosts title:string

This commit also moves the template the model generator uses for the
migration to the migration templates folder, as it seems a more
sensible place for it now that it is shared code.

Removed the bit about `SCRIPT_NAME` handling, since setting
`default_url_options[:script_name]` does work correctly.
We ran into an issue with it but it was an app bug.

Add some more documentation to the upgrade guide

[ci skip]