Fix adding implicitly rendered template digests to ETags

[javan]

Fixes that modifying an implicitly rendered template for a controller action using fresh_when or stale? would not result in a new ETag value.

New test failing without this change:

$ bundle exec rake test  TEST=test/controller/render_test.rb
Run options: --seed 9680

# Running:

.......S..........................F.........................

Finished in 0.212498s, 282.3556 runs/s, 1416.4839 assertions/s.

  1) Failure:
EtagRenderTest#test_etag_reflects_implicit_template_digest [.../actionpack/test/controller/render_test.rb:558]:
Expected response to be a <200: ok>, but was a <304: Not Modified>.
Expected: 200
  Actual: 304

60 runs, 301 assertions, 1 failures, 0 errors, 1 skips

/cc @jeremy

[jeremy]

❤️

[jeremy]

Squashed @ 893fc51 and backported to 5-0-stable @ 1955fa9