Do not include application.js if it doesn't exists

Rails were including 'application.js' to the pack when using
`javascript_include_tag :all` even there's no application.js in the
public directory.

add changelog for #7202

Add AS::StringInquirer#respond_to? method

Consistently with #method_missing

Make sure :environment task is executed before db:schema:load or
db:structure:load

Selecting which key extensions to include in active_support/rails
made apparent the systematic usage of Object#in? in the code base.
After some discussion in

    https://github.com/rails/rails/commit/5ea6b0df9a36d033f21b52049426257a4637028d

we decided to remove it and use plain Ruby, which seems enough
for this particular idiom.

In this commit the refactor has been made case by case. Sometimes
include? is the natural alternative, others a simple || is the
way you actually spell the condition in your head, others a case
statement seems more appropriate. I have chosen the one I liked
the most in each case.

Changing AR:CollectionAssociation#empty? to use #exists?

COUNT(*) queries can be slow in PostgreSQL, #exists? avoids this by
selecting a single record.

Remove warning: `*' interpreted as argument prefix

Passing options as the last value in an array doesn't work with form_for.
This reverts commit 61c8a4d9.

Passing options as the last value in an array doesn't work with form_for.
This reverts commit 6be564c7.

Refator route set

Allow perform_deliveries to be set within mailer action

Rather than keep the url options in record_or_hash_or_array, extract it
and reverse merge with options as it may contain important private keys
like `:routing_type`.

Closes #7259

Conflicts:
	activemodel/lib/active_model/secure_password.rb
	activerecord/lib/active_record/associations/collection_proxy.rb

This reverts commit 3fb0100e.

Update Migration Docs: Defaults for Boolean Fields

A Boolean field will accept true or false as defaults instead of 0 / 1.

Generating an URL with an array of records is now able to build a query
string if the last item of the array is a hash.

added info about an error that mysql gives when rails@localhost has a password; removed reference to activerecord/test/connections, which isn't there anymore

Closes #6600

calculate errors_options one time in validate_each

errors_options is calculated for every *CHECKS* but it could be
calculated one time  because options and RESERVED_OPTIONS
not change during this cycle