Pass 3 over testing guide

- Various grammar fixes
- Added assertions for update controller action tests
- Added user helper tests
- Fix typos

[ci skip]

typo fix [ci skip]

Spelling mistake - direcotry > directory

- Updated persistence documentation to make it clear that save and sa…

- Updated persistence documentation to make it clear that save and save! won't update a record if validation fails.
- Also fixed `update` method's documention to be uniform about this statement.
Fixes #20821

[ci skip]

[Vipul A M & pseidemann ]

This reverts commit ff835f90, reversing
changes made to c4d85dfb.

Reason: This broke the tests. We will add back after investigated.

Warn if `AR.primary_key` is called for a table who has composite prim…

Refactor `column_exists?` in `SchemaStatements`

Fix `bigint?` for Enum columns in MySQL

Make `SchemaMigration.drop_table` to one SQL

There is no need to define test if a connection does not support prim…

Remove unused `LOST_CONNECTION_ERROR_MESSAGES`

Explicitly define `columns` method as an interface

remove unused variable from render test

remove unused require

remove unnecessary gsub for `action_cable_meta_tag`

Refactor `OID::Money.precision`

Follow up to #22896.

`LOST_CONNECTION_ERROR_MESSAGES` was added by f3845822.
But currently unused from anywhere.

There are some cases when the `body` in `response_body=` can be set to
nil. One of those cases is in `actionpack-action_caching` which I found
while upgrading it for Rails 5.

It's not possible to run `body.each` on a `nil` body so we have to
return after we run `response.reset_body!`.

If the specified `skip_action_cable` option, so as not to output the
`action_cable_meta_tag` in template, gsub is unnecessary.

ref: https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/app/templates/app/views/layouts/application.html.erb.tt#L6..L8

`SchemaMigration.drop_table` is only used in tests.
Simply we can use `drop_table if_exists: true`.

`ActiveRecord::ConnectionAdapters::SchemaStatements#columns` is defined
here as an interface method here. So changes to raise `NotImplementedError`
same as `tables`, `views` ...etc.

If `AR.primary_key` is called for a table who has composite primary key,
the method returns `nil`. This behavior sometimes generates invalid SQL.
The first time developers notice to invalid SQL is when they execute
SQL. This commit enables developers to know they are doing something
dangerous as soon as possible.

`with_indifferent_access` had been used in `assigns` method, but has been removed in ca83436d.

This removes the following warning.

```
rails/actionpack/test/controller/render_test.rb:278: warning: assigned but unused variable - response
```

Add an after_bundle callback in Rails plugin templates

Add additional documentation on Headers#[] [ci skip]

The detach used by close! seems to be making EM very sad on Travis.

Issue #16519 covers confusion potentially caused by how HTTP
headers, that contain underscores in their names, are retrieved
through `ActionDispatch::Http::Headers#[]`.

This confusion has its origin in how a CGI maps HTTP header names
to variable names. Even though underscores in header names
are rarely encountered, they are valid according to RFC822 [1].
Nonetheless CGI like variable names, as requested by the Rack
specfication, will only contain underscores and therefore the
original header name cannot be recovered after the Rack server passed
on the environemnt hash. Please, see also the disscussion on
StackOverflow [2], which also links to an explaination in the
nginx documentation [3].

[1] http://www.ietf.org/rfc/rfc822.txt
[2] http://stackoverflow.com/questions/22856136/why-underscores-are-forbidden-in-http-header-names
[3] https://www.nginx.com/resources/wiki/start/topics/tutorials/config_pitfalls/#missing-disappearing-http-headers

It's not strictly necessary, and maybe this will help with the current
test failure.

remove require logging

EventMachine -> concurrent-ruby, take two