typos fixes
Tweak Configuring Guide language

the fact that Rails is multi-thread by default is not very relevant. see https://github.com/rails/rails/pull/13998 [ci skip]
:scissors:

This commit also addresses rails/docrails#169 and rails/rails#14159
Improve guide for Hash#transform_keys and related methods.

[skip ci]
Fix code alignment in duplicable? guide.

[skip ci]
Improve collision explanation for Hash#transform_keys.

Based on comments at
https://github.com/rails/docrails/commit/42417b0013819e573be9ffc5402e179003156e8e#commitcomment-5527251

[skip ci]
Cache key should be different when is Array.

`cache.fetch(['foo'])` and `cache.fetch('foo')` should generate
different cache keys as they are not equivalents.

[related #8615]
[related #8614]
Make exists? use bound values.

When we build a query with an inline value that is a numeric (e.g.
because it's out of range for an int4) PostgreSQL doesn't use an index
on the column, since it's now comparing numerics and not int4s.

This leads to a _very_ slow query.

When we use bound parameters instead of inline values PostgreSQL
raises numeric_value_out_of_range since no automatic coercion happens.
Allow string hash values on AR order method

This behavior has almost no performance impact:

String not allowed  66.910000   0.030000  66.940000 ( 67.024976)
String allowed      69.360000   0.030000  69.390000 ( 69.503096)

Benchmarked with http://git.io/Y0YuRw.
Merge pull request #14269 from arthurnn/expanded_key_array

Cache key should be different when is Array.
Revert "Merge pull request #14269 from arthurnn/expanded_key_array"

This reverts commit 475c96589ca65282e1a61350271c2f83f0d4044f, reversing
changes made to 705915ab5cf24430892107764b0050c07e1df583.

We decided that this is not worth busting everyone's cache as this
seems like a very unlikely problem. The problem only occurs when the
user is 1) not using a namespace, or 2) using the same namesapce for
different *kinds* of cache items. The recommended "fix" is to put
those cache items into their own namspace:

    id = 1
    Rails.cache.fetch(id, namespace: "user"){ User.find(id) }

    ids = [1]
    Rails.cache.fetch(ids, namespace: "users"){ User.find(ids) }

See the discussion on #14269 for details.
Merge pull request #14261 from MSch/bound-parameters-for-exists

Make exists? use bound values.
Merge pull request #14263 from robin850/allow_passing_string_to_order_hash

Follow up of #10732 - Allow string hash values on AR order method
quick pass over Active Record CHANGELOG. [ci skip].
docs, flash message keys are normalized to strings. [ci skip]

This is a follow up to a668beffd64106a1e1fedb71cc25eaaa11baf0c1
we only need to support `asc` and `ASC`. No need for mixed cases. #14263

This is a result of the discussion at https://github.com/rails/rails/pull/14263/files#r10291489
get rid of intermediate arrays.

origin: https://github.com/rails/rails/commit/f6aeb8b1a3687c8523e4a56309fe3736011b2935#commitcomment-5569649
Remove a reference to an issue [ci skip]

This actually not an issue fixing ; the changelog is referring to the
introducing pull request itself.
Do note remove `Content-Type` when `render :body`

`render :body` should just not set the `Content-Type` header. By
removing the header, it breaks the compatibility with other parts.

After this commit, `render :body` will returns `text/html` content type,
sets by default from `ActionDispatch::Response`, and it will preserve
the overridden content type if you override it.

Fixes #14197, #14238

This partially reverts commit 3047376870d4a7adc7ff15c3cb4852e073c8f1da.
Introduce `Rails.gem_version`

This method return `Gem::Version.new(Rails.version)`, suggesting a more
reliable way to perform version comparison.

Example:

    Rails.version #=> "4.1.2"
    Rails.gem_version #=> #<Gem::Version "4.1.2">

    Rails.version > "4.1.10" #=> false
    Rails.gem_version > Gem::Version.new("4.1.10") #=> true
    Gem::Requirement.new("~> 4.1.2") =~ Rails.gem_version #=> true

This was originally introduced as `.version` by @charliesome in #8501
but got reverted in #10002 since it was not backward compatible.

Also, updating template for `rake update_versions`.