Fixed CONTENT_LENGTH header in ActionController::TestRequest

Should except `:distinct` rather than `:order` for `exists?`

Updated Sprockets Documentation

Removes 'raw: true' from MemCacheStore#read_multi

use `force_encoding` instread of `encode!` to avoid `UndefinedConversionError`

Respect new records for `CollectionProxy#uniq`

The issue presented in #26246 showed a deeper underlying problem. When
we fell back to the exception handler for an exceptions cause, we were
calling that handler with the outer raised exception. This breaks the
calling code's expectations, especially if the exception has methods on
it behond those from `StandardError`.

Remove mention of deprecated SafeERB gem from security docs for now, prior section already speaks about sanitization as a safety measure. [ci skip] (#27086)

Fixes #27085

Fix mucking of connection_config leading to issues in prepared_statements issues

Fix PG prepared statement test

`PG::TextEncoder::Array#encode` returns the encoded value with `ASCII-8BIT`.
But in some cases, trying to convert `ASCII-8BIT` to `UTF-8` cause an error.

```ruby
"{\xE3\x83\x95\xE3\x82\xA1\xE3\x82\xA4\xE3\x83\xAB}".encode!(Encoding::UTF_8)
# => Encoding::UndefinedConversionError: "\xE3" from ASCII-8BIT to UTF-8
```
Should use `force_encoding` to avoid this error.

Follow up to 7ba3a48d

Ref: https://github.com/rails/rails/pull/23619#issuecomment-189924036

Enable JRuby testing for Active Job

All tests pass for me locally.

Use named parameters instead of `assert_valid_keys` in active_job/test_helper

Run tests by Ruby 2.2.6 and 2.3.2

Enable JRuby testing for Action Mailer

All tests passing locally for me:

```
195 runs, 441 assertions, 0 failures, 0 errors, 0 skips
```

The relation method `update_all` allows you to pass a SQL fragment. The
functionality is already mentioned in the prose but the examples section
does not cover it.

Fix testing isolation

Support AC::Parameters for PG HStore

As reported via #26904, there is a regression in how values for
Postgres' HStore column type are being processed, beginning in Rails 5.
Currently, the way that Active Record checks whether or not values need
to be serialized and put into the correct storage format is whether or
not it is a `Hash` object. Since `ActionController::Parameters` no
longer inherits from `Hash` in Rails 5, this conditional now returns
false. To remedy this, we are now checking to see whether the `value`
parameters being passed in responds to a certain method, and then
calling the `serialize` method, except this time with a real Hash
object. Keeping things DRY!

Fixes #26904.

AS::Testing::Isolation has two ways to isolate the process:
forking and subprocessing. The second way is used on JRuby and other
platforms that don't support forking.

The way how subprocessing works is that we prepare a command to run a
new process:

```
/opt/rubies/2.3.0/bin/ruby -I{skipped_load_path} test/initializable_test.rb '' -nInitializableTests::Basic#test_Initializer_provides_context's_class_name
```

As you see, there's unescaped quote at the end of the line.
It leads to:

```
sh: -c: line 0: unexpected EOF while looking for matching `''
sh: -c: line 1: syntax error: unexpected end of file
```

This fixes tests on MRI + NO_FORK variable and on JRuby 🎉

Refactor `NullRelation#calculate`

Before:

```ruby
    def calculate(operation, _column_name)
      if [:count, :sum].include? operation
        group_values.any? ? Hash.new : 0
      elsif [:average, :minimum, :maximum].include?(operation) && group_values.any?
        Hash.new
      else
        nil
      end
    end
```

After:

```ruby
    def calculate(operation, _column_name)
      case operation
      when :count, :sum
        group_values.any? ? Hash.new : 0
      when :average, :minimum, :maximum
        group_values.any? ? Hash.new : nil
      end
    end
```

CONENT_LENGTH setted by string length, which is equal to number of
characters in string but StringIO.length is byte sequence and
when payload contains non-ASCII characters, stream's length will be
different. That's why real byte length should be used for CONTENT_LENGTH
header.

Add unit test for CONTENT_LENGTH header fix

It just passes non-ascii symbols as parameters and verifies that
"CONTENT_LENGTH" header has content bytes count as value.

Without this, bundler tries to bundle json 1.8 on Ruby 2.4 in some cases
https://travis-ci.org/rails/rails/jobs/175874852

remove `alias_method_chain` from guide [ci skip]

Follow up to 7c848e6d

remove duplicated changelog entry [ci skip]

Fix small typo in Layouts and Rendering guide.

Changed "same way they do" to "same way they are".