Fix undef header method failures

```
  NoMethodError: undefined method `header' for #<Rack::MockResponse:0x00005611e9cbc950>
  Did you mean?  headers
```

`#header` was dropped in the Rack 3.1 release (https://github.com/rack/rack/blob/main/CHANGELOG.md#removed-1), while the `#location` helper has been around since 2.x (rack/rack@5e0b0a3)

test/resque-web_test.rb

@@ -307,7 +307,7 @@ module Test
   test 'redirects to overview' do
     post '/delayed/cancel_now'
     assert last_response.status == 302
-    assert last_response.header['Location'].include? '/delayed'
+    assert last_response.location.include? '/delayed'
   end
 end
 
@@ -316,7 +316,7 @@ module Test
 
   test 'redirects to delayed' do
     assert last_response.status == 302
-    assert last_response.header['Location'].include? '/delayed'
+    assert last_response.location.include? '/delayed'
   end
 end