A warning line should look like a warning section

[ci skip]
[ci skip] Add Doc of with_options for the case when inherited default options and original options have same keys
Use released rails-dom-testing
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.
Merge pull request #17292 from tricknotes/generator-test-doc

Fix example code for `Rails::Generators::Testing::Behaviour` [ci skip]
Clarify wording in Rails HTML Sanitizer section

[ci skip]
Merge pull request #17293 from djpowers/patch-1

Clarify wording in Rails HTML Sanitizer section [ci skip]
Merge pull request #15827 from yuki24/another-improvements-for-dynamic-error-pages

A warning line should look like a warning section in Guides
Improve the warning a bit [ci skip]
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.
Merge pull request #17296 from sgrif/sg-booleans-should-make-sense

Add a deprecation warning for abiguous boolean values
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'
```
Merge pull request #17300 from claudiob/add-necessary-require-reverse-merge

Add necessary 'require reverse_merge' to HAWI.rb
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
```
Merge pull request #17302 from claudiob/replace-slower-block-call-with-faster-yield

Replace (slower) block.call with (faster) yield
Merge pull request #17034 from pramod-sharma/master

[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
```
[ci skip] Add Doc of with_options for the case when inherited default options and original options have same keys
[ci skip] Add a note about headers overwriting

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
Merge branch 'master' of github.com:rails/rails