Replace the word «Accepts» by «Accept» in the example of custom request headers.

* Indent after private/protected
* Ruby >= 1.9 syntax for hashes
* Prefer method { do_stuff } instead of method{do_stuff} for single-line blocks.

[ci skip]

Improve the "Testing Rails Applications" guide adding a section for
testing helpers and a little note about associations with ActiveRecord
and YAML fixtures.

Also add a link to Fixture Builder ; yet another replacement for
fixtures such as Machinist or FactoryGirl.

Closes #9325.

I adjusted the example and the description in the testing guide and
simply linked from the mailer to the testing guide. This way we don't
have to maintain two separate places.

The env hash passed to `Http::Headers#new` must be in env format.
Also be aware that the passed hash is modified directly.

docs and test-cases for setting headers/env in functional tests.

Follow up to #9700.

Any flags that got set will be passed through to MiniTest::Unit.runner,
such as `-n`, `-s-, and `-v`.

* Update test invocation to use `rails test` instead.
* Update all the test command previews (since we're now using MiniTest.)
* Mentioning MiniTest instead of Test::Unit.
* Update list of test suites.

Update documentation for yml usage against https://github.com/rails/rails/issues/8612