Introduce config.action_mailer.default_from=

Allows to easily set :from, :replay_to, etc. options in
config/application.rb using simple syntax:

  config.action_mailer.default_options = {from:"no-replay@example.org"}

This was not possible using #default method because

  config.action_mailer.default(from: "no-replay@example.org")

is interpreated as reader method and just returns nil.
It would not call ActionMailer::Base.default method. The only
way of calling this method from config/application.rb was to use
the direct syntax which looks ugly in my opinion:

  config.assets.enabled = false
  config.assets.version = '1.0'
  config.encoding = "utf-8"
  config.action_mailer.default_url_options= {
    host:"example.org",
    protocol:"https"
  }
  ActionMailer::Base.default(from: "no-replay@example.org")
Remove mention to csv fixtures from test_helper in guides [ci skip]
Update release notes with AR::Relation#inspect change [ci skip]

See 07314e64fd62fb8e6165c8c539420160da9437e9.
Also fix some tabs in AR Changelog.
Disable query cache for lock queries

Fixes #867
Merge pull request #6985 from sidonath/disable-query-cache-for-locks

Disable query cache for lock queries
Limit the number of records in Relation#inspect

While it's interesting to have the results array, it can make a console or a webpage freeze if there are a lot of them.
So this limits the number of records displayed in #inspect to 10 and tells how much were effectively found.
Prevent conflict between mime types and Object methods

Assuming the type ":touch", Collector.new was calling
send(:touch), which instead of triggering method_missing
and generating a new collector method, actually
invoked the private method `touch` inherited from
Object.

By generating the method for each mime type as it
is registered, the private methods on Object can
never be reached by `send`, because the `Collector`
will have them before `send` is called on it.

To do this, a callback mechanism was added to Mime::Type

This allows someone to add a callback for whenever
a new mime type is registered. The callback then
gets called with the new mime as a parameter.

This is then used in AbstractController::Collector
to generate new collector methods after each mime
is registered.
Merge pull request #6990 from mirceapricop/bug/mime-type-method-name

Fix: Mime type names conflict with Object methods
Added failing test for generating namespaced table_name_prefix in engines
Fixed generating namespaced table_name_prefix in engines
update test_help to config properly turn natural language option

Last versions of Turn don't monkey patch MiniTest to setup
the natural language option. Here is an
[example](https://github.com/TwP/turn/blob/master/try/test_autorun_minitest.rb#L3).

This patches the following behaviour:

    $ rake test:units
    `<top (required)>': undefined method `use_natural_language_case_names='
    for MiniTest::Unit:Class (NoMethodError)
Merge pull request #6995 from frodsan/update_test_help

update test_help to config properly turn natural language option
Merge pull request #6950 from paneq/default_from2

Introduce config.action_mailer.default_options=

Allows to easily set :from, :replay_to, etc. options in
config/application.rb using simple syntax:

    config.action_mailer.default_options = { from: "no-replay@example.org" }

Closes #6747
Improve docs, changelog and release notes for Action Mailer default_options=

[ci skip]
Fix failing AM test due to missing template
Remove unused code.
Added *instance_writer: false* for stored/serialized attributes.
Merge pull request #6987 from dmathieu/limit_inspect

Limit the number of records in Relation#inspect
Update master
Merge pull request #6997 from kennyj/fix_stored_attributes

Added *instance_writer: false* to stored/serialized attributes.