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]

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...

It doesn't make sense for the subclass to implement this method, and not
have it on the parent. We can also DRY up the implementation of
`#lookup` to be defined in terms of fetch, which will give us a single
point of entry

Remove bullet point to unwritten/unreferenced section

This reverts commit da99a2a2.

Support symbol foreign key to delete

Add space in :start_year description for date_select

We added a comparison to "id", and call to `self.class.primary_key` a
*lot*. We also have performance hits from `&block` all over the place.
We skip the check in a new method, in order to avoid breaking the
behavior of `read_attribute`