fix wrong example from #17318 [ci skip]

make guides example more friendly to nonlatin langs [ci skip]

[ci skip] add :queue option to deliver_later and deliver_later! documentation

Replace (slower) block.call with (faster) yield

This reverts commit 0ab075e7.

Since some headers can appear multiple times in an email it's required
to set it to nil first when you want to overwrite an existing one.

This commit add some information about this process.

Fix #15912

[ci skip] Add Doc of with_options for the case when inherited default options and original options have same keys

Performance optimization: `yield` with an implicit `block` is faster than `block.call`.
See http://youtu.be/fGFM_UrSp70?t=10m35s and the following benchmark:

```ruby
require 'benchmark/ips'

def fast
 yield
end

def slow(&block)
 block.call
end

Benchmark.ips do |x|
 x.report('fast') { fast{} }
 x.report('slow') { slow{} }
end

# => fast    154095 i/100ms
# => slow     71454 i/100ms
# =>
# => fast  7511067.8 (±5.0%) i/s -   37445085 in   4.999660s
# => slow  1227576.9 (±6.8%) i/s -    6145044 in   5.028356s
```

[ci skip] Add Doc of with_options for the case when inherited default options and original options have same keys

Replace (slower) block.call with (faster) yield

Performance optimization: `yield` with an implicit `block` is faster than `block.call`.
See http://youtu.be/fGFM_UrSp70?t=10m35s and the following benchmark:

```ruby
require 'benchmark/ips'

def fast
 yield
end

def slow(&block)
 block.call
end

Benchmark.ips do |x|
 x.report('fast') { fast{} }
 x.report('slow') { slow{} }
end

# => fast    154095 i/100ms
# => slow     71454 i/100ms
# =>
# => fast  7511067.8 (±5.0%) i/s -   37445085 in   4.999660s
# => slow  1227576.9 (±6.8%) i/s -    6145044 in   5.028356s
```

Add necessary 'require reverse_merge' to HAWI.rb

Hashes with indifferent access should support `reverse_merge` out-of-the-box
but they don't; for instance the following code fails:

```ruby
require 'active_support'
require 'active_support/hash_with_indifferent_access'
hash = HashWithIndifferentAccess.new key: :old_value
hash.reverse_merge key: :new_value
```

This PR fixes the case above by simply requiring
`active_support/core_ext/hash/reverse_merge` in `hash_with_indifferent_access.rb`
and adding a test that confirms the fix.

---

Here are more details about the bugfix.
Currently, `reverse_merge` is [defined in HashWithIndifferentAccess](https://github.com/rails/rails/blob/4e8ea13ba1a0870905a46fac5f232d9f41eef8a4/activesupport/lib/active_support/hash_with_indifferent_access.rb#L208)
by invoking `super`, that is by invoking `Hash#reverse_merge`:

```ruby
def reverse_merge(other_hash)
  super(self.class.new_from_hash_copying_default(other_hash))
end
```

However, Ruby's `Hash` does not have the `reverse_merge` by default: it must be
added by ActiveSupport, and that requires the following line of code to be
present:

```ruby
require 'active_support/core_ext/hash/reverse_merge'
```

Add a deprecation warning for abiguous boolean values

In Rails 5.0, we'd like to change the behavior of boolean columns in
Rails to be closer to Ruby's semantics. Currently we have a small set
of values which are "truthy", and all others are "falsy". In Rails 5.0,
we will reverse this to have a small number of values which are "falsy",
and all others will become "truthy".

In the interim, all values which are ambiguous must emit a deprecation
warning.

A warning line should look like a warning section in Guides

Clarify wording in Rails HTML Sanitizer section [ci skip]

[ci skip]

Fix example code for `Rails::Generators::Testing::Behaviour` [ci skip]

`cleanup_destination_root` method is not found anywhere.
Instead, `prepare_destination` clean up distination root on setup.

Closes #17197.
Closes #17207.

`{required}` is a type modifier so it should be:

    user:references{required}:index

and not:

    user:references:index{required}

mention the :without option [ci skip]

As pointed out in the ActiveModel::Validations::HelperMethods #validates_format_of documentation.

clarify debugger platform invocation

The behavior has changed since 4.1 and non-array values are no
longer type casted to a blank array. This way the user can define
custom validations on that property.

[ci skip] add AC::InvalidCrossOriginRequest to list of rescue_responses default

/cc @sgrif

remove unneeded file from Railties.