* it's Action Mailer not ActionMailer
* it's Action Pack not ActionPack
* it's Active Support not ActiveSupport

/cc @fxn

/cc @fxn

Added documentation for beginning_of_minute and end_of_minute to Active Support Core Extensions guide

instance_variable_names remains for internal use, but it's not that useful for the users anymore

Don't use:
* Time.time_with_datetime_fallback
* Time.utc_time
* Time.local_time

There's no reason for guides to reference old behaviors. They should
be current as of the versions of Rails that they ship with, and including
older information just clutters thing.

I discussed this change with @fxn and he agrees.

Patched Marshal#load to work with constant autoloading (active_support/dependecies.rb) (issue #8167)

ActiveRecord::Observer was extracted into a separate gem so it
should not be referenced anymore (see https://github.com/rails/rails/commit/ccecab3)

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

Previously this code just assumed it is capable of changing the file
ownership, both user and group. This will fail in a lot of scenario's
unless:
* The process is run as a superuser (root);
* The owning user and group are already set to the user and group we're
  trying to chown to;
* The user chown'ing only changes the group to another group it is a
  member of.
If either of those conditions are not met the filesystem will simply
deny the operation throwing an error.

It is also not always possible to do a chmod, there might be a SELinux
policy or another limitation preventing the user to change the file
mode. To this end the chmod call has also been added to the rescue
block.

I've also added a little comment above the chmod command that doing a
chmod on a file which has an ACL set will cause the ACL to be
recalculated / modified.

This reverts commit f5b9ed4f.

REASON: Translation work can not be done in the docrails repository

The start of the week is a parameter nowadays, it is a little
confusing that we say here that we assume something about it.
Better to use "next" and "previous", which are clear regardless
of the concept of beginning of week.

Adds examples for the edge cases where the receiver is a
Monday or Sunday itself.

This reverts commit abf8de85.
We should take a deeper look to those cases flat_map doesn't do deep
flattening.

irb(main):002:0> [[[1,3], [1,2]]].map{|i| i}.flatten
=> [1, 3, 1, 2]
irb(main):003:0> [[[1,3], [1,2]]].flat_map{|i| i}
=> [[1, 3], [1, 2]]