Fix test cases for inflector.rb

`yield` was missing in `with_dup`, due to which 57 assertions were not running in real, and all tests were passing by default.

Error was introduced in commit - 31ceb5e.

```ruby
# Test case result for inflector.rb
#  before
    445 runs, 1160 assertions, 0 failures, 0 errors, 0 skips
#  after
    445 runs, 1217 assertions, 0 failures, 0 errors, 0 skips
```