Cache check if `default_scope` has been overridden.

Benchmark Script:
```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  # gem 'rails', github: 'rails/rails', ref: 'f1f0a3f8d99aef8aacfa81ceac3880dcac03ca06'
  gem 'rails', path: '~/rails'
  gem 'arel', github: 'rails/arel', branch: 'master'
  gem 'rack', github: 'rack/rack', branch: 'master'
  gem 'sass'
  gem 'sprockets-rails', github: 'rails/sprockets-rails', branch: 'master'
  gem 'sprockets', github: 'rails/sprockets', branch: 'master'
  gem 'pg'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('postgres://postgres@localhost:5432/rubybench')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

attributes = {
  name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
  email: "foobar@email.com",
}

1000.times { User.create!(attributes) }

Benchmark.ips(5, 3) do |x|
  x.report('where with hash') { User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.report('where with string') { User.where("users.name = ?", "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.compare!
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.")
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"
```

Stackprof output truncated.
```
TOTAL    (pct)     SAMPLES    (pct)     FRAME
   52  (10.6%)          10   (2.0%)     ActiveRecord::Scoping::Default::ClassMethods#build_default_scope
```

Before:
```
Calculating -------------------------------------
     where with hash     2.789k i/100ms
   where with string     4.407k i/100ms
-------------------------------------------------
     where with hash     29.170k (± 1.9%) i/s -    147.817k
   where with string     46.954k (± 2.7%) i/s -    237.978k

Comparison:
   where with string:    46954.3 i/s
     where with hash:    29169.9 i/s - 1.61x slower

Total Allocated Object: 85
Calculating -------------------------------------
                 all    16.773k i/100ms
-------------------------------------------------
                 all    186.102k (± 3.6%) i/s -    939.288k
```

After:
```
Calculating -------------------------------------
     where with hash     3.014k i/100ms
   where with string     4.623k i/100ms
-------------------------------------------------
     where with hash     31.524k (± 1.3%) i/s -    159.742k
   where with string     49.948k (± 2.3%) i/s -    249.642k

Comparison:
   where with string:    49948.3 i/s
     where with hash:    31524.3 i/s - 1.58x slower

Total Allocated Object: 84
Calculating -------------------------------------
                 all    20.139k i/100ms
-------------------------------------------------
                 all    227.860k (± 2.5%) i/s -      1.148M
```

0 files changed, 0 insertions, 0 deletions