improve performance of integration tests.

I found delegate to be a bottleneck during integration tests.  Here is
the test case:

```ruby
require 'test_helper'

class DocumentsIntegrationTest < ActionDispatch::IntegrationTest
  test "index" do
    get '/documents'
    assert_equal 200, response.status
  end
end

Minitest.run_one_method(DocumentsIntegrationTest, 'test_index')
StackProf.run(mode: :wall, out: 'stackprof.dump') do
  3000.times do
    Minitest.run_one_method(DocumentsIntegrationTest, 'test_index')
  end
end
```

Top of the stack:

```
[aaron@TC integration_performance_test (master)]$ stackprof stackprof.dump
==================================
  Mode: wall(1000)
  Samples: 23694 (7.26% miss rate)
  GC: 1584 (6.69%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
      7058  (29.8%)        6178  (26.1%)     block in Module#delegate
       680   (2.9%)         680   (2.9%)     ActiveSupport::PerThreadRegistry#instance
       405   (1.7%)         405   (1.7%)     ThreadSafe::NonConcurrentCacheBackend#[]
       383   (1.6%)         383   (1.6%)     Set#include?
       317   (1.3%)         317   (1.3%)     ActiveRecord::Base.logger
       281   (1.2%)         281   (1.2%)     Rack::Utils::HeaderHash#[]=
       269   (1.1%)         269   (1.1%)     ActiveSupport::Notifications::Fanout::Subscribers::Evented#subscribed_to?
       262   (1.1%)         262   (1.1%)     block (4 levels) in Class#class_attribute
       384   (1.6%)         246   (1.0%)     block (2 levels) in Class#class_attribute
```

According to @eileencodes's tests, this speeds up integration tests so
that they are only 1.4x slower than functional tests:

Before:

  INDEX: Integration Test:      153.2 i/s - 2.43x slower

After:

  INDEX: Integration Test:      275.1 i/s - 1.41x slower

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