Fix inflector test by using dup inflections when it needs to be changed

This is not something that is failing at the moment, but can do it
eventually. I had the issue with db62081 as the HEAD and with the
following change:

```
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -101,6 +101,7 @@ class InflectorTest < ActiveSupport::TestCase
   def test_acronyms
     ActiveSupport::Inflector.inflections do |inflect|
       inflect.acronym("API")
+      inflect.acronym("HTM")
       inflect.acronym("HTML")
       inflect.acronym("HTTP")
       inflect.acronym("RESTful")
```

I was expecting only `test_acronyms` to fail, but with a specific `seed`
others were also failing: `ruby -w -I"lib:test" test/inflector_test.rb
--seed 4313`.

Now, `inflections` instance is duplicated on `setup` and restored on
`teardown`.

I decided to benchmark and check the impact of the patch and it seems
to me to be fine.

```
Calculating -------------------------------------
     without changes     1.000  i/100ms
      with setup dup     1.000  i/100ms
      with block dup     1.000  i/100ms
-------------------------------------------------
     without changes      0.817  (± 0.0%) i/s -      5.000  in
     6.119916s
      with setup dup      0.784  (± 0.0%) i/s -      4.000
      with block dup      0.797  (± 0.0%) i/s -      4.000
```

Where `with setup dup` duplicates on setup for each test and `with block`
duplicates for just for tests that actually modify `inflections`.

0 files changed, 0 insertions, 0 deletions