Introduce `ActionView::Template::Handlers::ERB.escape_whitelist`

This is a list of mime types where template text is not html escaped
by default. It prevents `Jack & Joe` from rendering as
`Jack & Joe` for the whitelisted mime types. The default whitelist
contains text/plain.

This follows a whitelist approach where plain text templates are
not escaped, and all the others (json, xml) are. The mime type is
assumed to be set by the abstract controller.

Encrypted cookies

Make XmlMini.with_backend usable with threads

Conflicts:
	activesupport/CHANGELOG.md

`XmlMini.with_backend` now may be safely used with threads:

  Thread.new do
    XmlMini.with_backend("REXML") { rexml_power }
  end
  Thread.new do
    XmlMini.with_backend("LibXML") { libxml_power }
  end

Each thread will use it's own backend.

The method #remove_const does not load the file, so we
can still remove the constant.

Avoid using Integer#/, as it is redefined by the 'mathn' stdlib

Use BCrypt's MIN_COST in the test environment for speedier tests

[ci skip] Correct examples for form_tag helper.

Conflicts:
	actionpack/lib/action_view/helpers/form_tag_helper.rb

Rake test:uncommitted finds git directory in ancestors.

Nothing should be raised anyway 😄

Thanks @spastorino ❤
https://github.com/rails/rails/pull/8202/files#r2112067

Regression test for #7238

`#pluck` can be used on a relation with `select` clause (#7551)

Add a regression test on #8195

Simplify code by taking advantage of latest mocha (v0.13.0).