`:numpad9` is duplicated instead of `:numpad8`, so `send_keys` method raises an error when it receives `:numpad8`.
We can reproduce this behaviour with the added test case.
This pull request fixes the problem.
Note
===
I found this bug with `RUBYOPT=-w`. For example: (The second warning)
```
$ ruby -Ilib -rcapybara-webkit -e ''
/path/to/lib/ruby/gems/2.6.0/gems/addressable-2.5.2/lib/addressable/idna/pure.rb:154: warning: assigned but unused variable - startercc
/path/to/lib/capybara/webkit/node.rb:217: warning: duplicated when clause is ignored
```
41c4306267