now we only have to look up @scope[:scope_level] once per call to
canonical_action? and we don't have a variable named "flag"

since we pass `as` down, then we won't have to do an insert / delete
dance with the options hash

UnexpectedError exceptions wrap the original exception, and the original
exception may contain a reference to something that can't be marshal
dumped which will cause the process to die.

[ci skip] add note about the ERB escape in generator docs

[ci skip] correct default cache store class

This solves errors like:

```
BaseTest#test_you_can_register_multiple_observers_to_the_mail_object_that_both_get_informed_on_email_delivery:
Errno::ECONNREFUSED: Connection refused - connect(2) for "localhost" port 25
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:541:in `initialize'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:541:in `open'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:541:in `tcp_socket'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:551:in `block in do_start'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/timeout.rb:91:in `block in timeout'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/timeout.rb:101:in `call'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/timeout.rb:101:in `timeout'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:550:in `do_start'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:520:in `start'
    /Users/senny/Projects/rails/.bundle/gems/mail-2.6.1/lib/mail/network/delivery_methods/smtp.rb:112:in `deliver!'
    /Users/senny/Projects/rails/.bundle/gems/mail-2.6.1/lib/mail/message.rb:2136:in `do_delivery'
    /Users/senny/Projects/rails/.bundle/gems/mail-2.6.1/lib/mail/message.rb:232:in `block in deliver'
    /Users/senny/Projects/rails/actionmailer/lib/action_mailer/base.rb:528:in `block in deliver_mail'
    /Users/senny/Projects/rails/activesupport/lib/active_support/notifications.rb:164:in `block in instrument'
    /Users/senny/Projects/rails/activesupport/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
    /Users/senny/Projects/rails/activesupport/lib/active_support/notifications.rb:164:in `instrument'
    /Users/senny/Projects/rails/actionmailer/lib/action_mailer/base.rb:526:in `deliver_mail'
    /Users/senny/Projects/rails/.bundle/gems/mail-2.6.1/lib/mail/message.rb:232:in `deliver'
    /Users/senny/Projects/rails/actionmailer/test/base_test.rb:598:in `block (2 levels) in <class:BaseTest>'
    /Users/senny/Projects/rails/actionmailer/test/base_test.rb:801:in `mail_side_effects'
    /Users/senny/Projects/rails/actionmailer/test/base_test.rb:593:in `block in <class:BaseTest>'
```
where the `:smtp` delivery method was leaked over to other test cases.

[ci skip] fix spelling of overridden

[ci skip] Updated where scope documentation to conform to new style

[Yves Senn & Matthew Draper]

The column check was embodied in the defaul index name.
If the :name option was used, the specified columns were not verified at all.

Given:

```
assert connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_yo_momma)
```

That index could have been defined on any field, not necessarily on `:foo_id`.

[ci skip] use "based on" or "based off of"

This was added back in Rails 3 on
c4a61092, and is not being used anymore.

Calling ActiveSupport::TestCase.i_suck_and_my_tests_are_order_dependent! in AS::TestCase makes
everyone's tests order dependent, which should never be done by the framework.

[ci skip] Fix broken url in plugins guide

Merge pull request #16456 from lsylvester/improve-active-record-error-message-on-update_columns-for-destroyed-records

Change error message in update_columns for destroyed objects

Mention changes to `nil` handling in serialized attributes

Cleanup Guides Guidelines [ci skip]

* Copy editing for clarity in several locations
* Rename "Titles" section to "Headings"
* Fix incorrect anchor in link to API Documentation Guidelines for filenames

[ci skip] Fix sample code in engines guide

[ci skip] Changed options to +options+ for rdoc

Pass block for logging.