`.reflections` public API changed to return a String instead of a Symbol
as keys.

see commit 1f314884 and 6259e4e2

[fixes #16928]
[fixes #17610]

For some reason changing `.find` to `.unscoped.find` in
https://github.com/rails/rails/commit/f766abd4cf3eb75469d3646cfb6d85e668c619f3
caused `scoping` to leak in some tests when run in isolation (looks like
a concurrency issue?). `relation_scoping_test.rb` is a case that failed.
From what I can tell it should not be possible, but changing to the
block form fixes it. There is a deeper issue that I can't seem to find.
/cc @senny

Also checked to make sure this does not affect foreign key constraints.
(It doesn't).

Fixes #12856
Closes #14088

They were introduced in 23b6def0; the serial stuff has been removed since a5d80f84

Fixing wrong link in 'Ruby on Rails Security Guide' [ci skip]

The URL

http://www.h-online.com/security/Symantec-reports-first-active-attack-on-a-DSL-router--/news/102352

points to an article titled 'The H is closing down'. The good one is:

http://www.h-online.com/security/news/item/Symantec-reports-first-active-attack-on-a-DSL-router-735883.html

[ci skip] correct example output and filename in pg guide

We can't change the signature without a deprecation cycle.

Move 'the' outside of SQLite and Bundler link text [ci skip]

Fix select_tag generating tag when set to false.

Prior to this patch you'd end up with an error like:

```
ActiveRecord::RecordNotFound: Couldn't find <Model> with 'id'=<id> [WHERE (<default_scope condition>)]
```

Update grammar in Getting Started Guide [ci skip]

Wrap code snippets in +, not backticks, in sdoc [ci skip]

Fixes https://github.com/rails/rails/issues/17701.

We generate a `.keep` file inside the log directory to make
sure the directory itself is under version control. let's keep
it that way.

/cc @matthewd

why not gitignore /log as a whole?

[ci skip]

I grepped the source code for code snippets wrapped in backticks in the comments
and replaced the backticks with plus signs so they are correctly displayed in
the Rails documentation.

[ci skip]

Wrap code snippets in +, not backticks, in sdoc [ci skip]

[ci skip]

Use request_method instead of method in instrumentation

Document *all* the options accepted by form_for

Conflicts:
	actionview/lib/action_view/helpers/form_helper.rb

_will_change! method is not needed any more [ci skip]

Add `:enforce_utf8` option to form_for

ref #15674

Since 06388b07 `form_tag` accepts the option `enforce_utf8` which, when set to
false, prevents the hidden "UTF8 enforcer" field from appearing in the output.

This commit implements the same behavior for `form_for`.

Stems from https://github.com/rails/rails/pull/17685#issuecomment-63871395

The [current documentation of form_for](http://api.rubyonrails.org/classes/ActionView/Helpers/FormHelper.html#method-i-form_for) is incomplete:

> The rightmost argument to form_for is an optional hash of options -
>   :url - The URL the form is to be submitted to. This may be represented in the same way as values passed to url_for or link_to. So for example you may use a named route directly. When the model is represented by a string or symbol, as in the example above, if the :url option is not specified, by default the form will be sent back to the current url (We will describe below an alternative resource-oriented usage of form_for in which the URL does not need to be specified explicitly).
>   :namespace - A namespace for your form to ensure uniqueness of id attributes on form elements. The namespace attribute will be prefixed with underscore on the generated HTML id.
>   :html - Optional HTML attributes for the form tag.

This commit completes the documentation by specifying that:

* `:authenticity_token` is also a valid option (as [documented here](https://github.com/rails/rails/blob/b0f44c9ff0be84ebc42760029adeeb9dd954655f/actionview/lib/action_view/helpers/form_helper.rb#L396))
* `:method` is also a valid option (as [tested here](https://github.com/rails/rails/blob/b0f44c9ff0be84ebc42760029adeeb9dd954655f/actionview/test/template/form_helper_test.rb#L1819))
* `:remote` is also a valid option (as [tested here](https://github.com/rails/rails/blob/b0f44c9ff0be84ebc42760029adeeb9dd954655f/actionview/test/template/form_helper_test.rb#L1850))

[ci skip]

Use request method instead of ActionDispatch::Request#request_method instead of ActionDispatch::Request#method to pick up overrides by the middleware

[CI SKIP] Improvements to Active Job guide.

PERF: avoid string allocations

This makes the following changes:
  * warn if `:null` is not passed to `add_timestamps`
  * `timestamps` method docs link to `add_timestamps` docs
  * explain where additional options go
  * adjust examples to include `null: false` (to prevent deprecation warnings)

Now ActionView accepts <%= foo(){ %> and <%= foo()do %> ⛳

Allow class_name option in habtm to be consistent with other association...