Fix spurious warnings in tests when using Rack 3.1

These warnings are of the form:

```
warning: input stream used for POST parsing different from current input
stream. Starting in Rack 3.2, Rack will used the cached POST value
instead of parsing the current in put stream.
```

These warnings are spurious, because tests don't fail on the rack
master branch.  They occur because the requests inside the FAKE_APP
used in the tests have rack.input as a Rack::Lint:InputWrapper
instance, while the InputRewinder used in the tests replaces this
with the original instance after receiving the response.  Then when
last_request.POST is called after the request, you get the warning
because rack.request.form_input doesn't match rack.input.

Deleting rack.request.form_hash from the environment fixes this.
The tests that call last_request.POST will reparse the input body
instead of using the cached input, but that doesn't seem like a
problem.

spec/fixtures/fake_app.rb

@@ -143,6 +143,7 @@ def call(env)
         if input
           input.rewind
           env['rack.input'] = input
+          env.delete('rack.request.form_hash')
         end
       end
     end