File.exist? instead of File.exists? in bin/setup

File.exists? is deprecated in Ruby 2.1+

https://github.com/ruby/ruby/blob/v2_1_2/file.c#L1413
Add missing colon to internal
Remove unused `initialize_attributes` method

This was previously a hook for a special case related to `serialize`,
which has since been removed.
Merge pull request #15473 from sgrif/sg-missing-colon

Add missing colon to internal [ci skip]
Merge pull request #15474 from unixmonkey/master

File.exist? instead of File.exists? in bin/setup
Fix typo in ActionController Overview

* Should say "sensitive" instead of "sensible"
Merge pull request #15477 from aripollak/actioncontroller-overview-typo

Fix typo in ActionController Overview
concat is a hotspot (via AV#append=), so just directly define the methods
Merge pull request #15475 from sgrif/sg-initialize-attributes

Remove unused `initialize_attributes` method
reduce AS::SafeBuffer allocations

html_escape_interpolated_argument is only used in mutation methods:

  https://github.com/rails/rails/blob/c07d09559ec171e1904b55c7ad7e8c7d586ca51b/activesupport/lib/active_support/core_ext/string/output_safety.rb#L174
  https://github.com/rails/rails/blob/c07d09559ec171e1904b55c7ad7e8c7d586ca51b/activesupport/lib/active_support/core_ext/string/output_safety.rb#L179

The return value doesn't need to be converted to an AS::SafeBuffer since
we know that the current object is an AS::SafeBuffer and will be
mutated, and the return value from html_escape_interpolated_argument
will be thrown away
drastically reduce object allocations

before this change, we were allocating AS::SafeBuffer objects that were
being interpolated in to a string, so the safe buffer object was being
thrown away.  This change only allocates a string (vs a string *and* a
safebuffer) and interpolates the string.

On my test application, this reduced the AS::SafeBuffer objects from
1527k per request to about 500 per request.
call `capture` fewer times from `form_for`
fix polymorphic? method and reuse it

Fix polymorphic to check for `options[:polymorphic]` instead of
`options.key? :polymorphic` and then reuse the method `polymorphic?`
method instead of constantly checking the same `options[:polymorphic]`.
Merge pull request #15483 from eileencodes/reuse-available-reflection-polymorphic-methods

fix polymorphic? method and reuse it
pg test, examples for default values and schema dumping of point types.

/cc @sgrif
pg, preserve point type when schema dumping.
test pg, we don't care about the internal state of `column#default`.

That data is internal to Active Record. What we care about is that
new records have the right default value.
test pg, move bit string type tests into `bit_string_test.rb`.
Refactor determination of whether the field has changed

The types know more about what is going on than the dirty module. Let's
ask them!
pg, preserve type when schema dumping bit and bit varying columns.