Bug when visiting a page with params that contains square brackets

When I visit a page with square brackets in the params of the url like `/foo?bar[baz]=true`
Then I should see that page

Instead, the request is never sent.

Traced this as far as `lib/capybara/webkit/browser.rb:145` in the `command` method.

Run with the following:

```
rspec spec/integration/session_spec.rb --tag wip
```

found this with @coffeencoke