Fix broken test case

[fatkodima]

$ rake test
Run options: --seed 64620

# Running:

.........................F

Failure:
ActionCacheTest#test_explicit_html_format_is_used_for_fragment_path [/Users/fatkodima/Desktop/oss/actionpack-action_caching/test/caching_test.rb:817]:
Expected: nil
  Actual: "1662930799.070181<div />"


rails test Users/fatkodima/Desktop/oss/actionpack-action_caching/test/caching_test.rb:803

............

Finished in 0.509232s, 74.6222 runs/s, 349.5460 assertions/s.
38 runs, 178 assertions, 1 failures, 0 errors, 0 skips
rake aborted!
Command failed with status (1)

This test case had a bug and correctly started to fail after the rails/rails#43735.

So before that Rails' PR:

1. @cache_this instance variable variable was not reset after the first get in that test case
2. after the second get, the action was not executed and content_to_cache returned a stale @cache_this and compared it to the cached response from the action

After the Rails' PR:

1. @cache_this is reset to nil before the second get
2. content_to_cache return nil from the step 1, but cached response is not equal to nil, so here is the test failure

Having cached_time = content_to_cache line is incorrect and this bug was unnoticed before that PR, so we need to remove it.