Reference #17453

[Godfrey Chan, Washington Luiz]

Anchor should not be appended when set to nil/false.

[ci skip] Fix comment of ActionDispatch::Callbacks

cc ddce3dd3

cc #17719

Fix explanation for ActionDispatch::Callbacks. [ci skip]

ActionDispatch::Callbacks dose not run the prepare callbacks,
so change with comment on ActionDispatch::Callbacks.

Fixes 16928, improve reflections public API doc [ci skip]

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

see commit 1f314884 and 6259e4e2

[fixes #16928]
[fixes #17610]

Fixes https://github.com/rails/rails/issues/17714.

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