fix warnings when eager loading on ruby 2.7
before:
```
ruby -w -Itest -Ilib -I../activesupport/lib -I../actionpack/lib
-I../actionview/lib -I../activemodel/lib
test/application/loading_test.rb
Run options: --seed 11862
............/home/hartley/dev/github.com/skipkayhil/rails/activesupport/lib/active_support/dependencies/autoload.rb:75:
warning: instance variable @_eagerloaded_constants not initialized
/home/hartley/dev/github.com/skipkayhil/rails/activesupport/lib/active_support/dependencies/autoload.rb:75:
warning: instance variable @_eagerloaded_constants not initialized
.../home/hartley/dev/github.com/skipkayhil/rails/activesupport/lib/active_support/dependencies/autoload.rb:75:
warning: instance variable @_eagerloaded_constants not initialized
/home/hartley/dev/github.com/skipkayhil/rails/activesupport/lib/active_support/dependencies/autoload.rb:75:
warning: instance variable @_eagerloaded_constants not initialized
/home/hartley/dev/github.com/skipkayhil/rails/activesupport/lib/active_support/dependencies/autoload.rb:75:
warning: instance variable @_eagerloaded_constants not initialized
...
Finished in 2.819071s, 6.3851 runs/s, 13.1249 assertions/s.
18 runs, 37 assertions, 0 failures, 0 errors, 0 skips
```
after:
```
ruby -w -Itest -Ilib -I../activesupport/lib -I../actionpack/lib
-I../actionview/lib -I../activemodel/lib
test/application/loading_test.rb
Run options: --seed 34841
..................
Finished in 3.407082s, 5.2831 runs/s, 10.8597 assertions/s.
18 runs, 37 assertions, 0 failures, 0 errors, 0 skips
```
afce06bdb2