Merge pull request #17462 from vipulnsward/actionmailer-doc-fix

AM README fixes [ci skip]
Merge pull request #17305 from ziggythehamster/activerecord-connectionhandling-RAILS_ENV-without-rails

If Rails is not defined, check ENV["RAILS_ENV"] and ENV["RACK_ENV"] in ActiveRecord::ConnectionHandling
Merge pull request #17040 from clyfe/patch-1

Explicit route test class
Load test schema even if there are no migrations

Fixes #17170
Don't modify the columns hash to set defaults from the attributes API

Nothing is directly using the columns for the default values anymore.
This step helps us get closer not not mutating the columns hash.
Don't attempt to load the schema file in tests if none exists
added punctuations, and role of queue_adapter module [ci skip]
Remove redundant substitute index when constructing bind values

We end up re-ordering them either way when we construct the Arel AST (in order
to deal with rewhere, etc), so we shouldn't bother giving it a number in the
first place beforehand.
Added region sequencing of primary keys for Postgres.

Skip setting sequence on a table create if the value is 0 since it will start the first value at 1 anyway.
This fixes the PG error 'setval: value 0 is out of bounds for sequence vms_id_seq...' encountered when migrating a new DB.

BugzID: 15452,9772,13475,16850
fix set_pk_sequence and add a test for it.
remove unused and untested API
Fix break on generated html

fixes this :

![http://i.imgur.com/86scI3a.png](http://i.imgur.com/86scI3a.png))
Merge pull request #17470 from keikun17/master

[ci skip] Fix line break on asset pipeline guide
placed description at correct place [ci skip]
- Remove unused duplicated method `add_column_position` from AbstractMysqlAdapter
development.rb should mention available log levels
add missing space.[ci skip]
Merge pull request #17474 from kuldeepaggarwal/fix-indentaion

add missing space.[ci skip]
Tiny documentation fixes and styling improvements

[ci skip]
Remove redundant require of file

This file was required inside 'test/validators/namespace/email_validator.rb'
that's already required here. Therefore I removed the redundant required.