Timed out requests or Net::TimeoutError when using FastBoot

I've been noticing some consistent test failures with my Selenium test suite recently but they all go away if I run them along with `DISABLE_FASTBOOT=true`.

Strangely, the test failures only occur with RSpec tests that are using file fixtures.

My hunch is that maybe my use of `deferRendering` in a `setupController` hook is causing FastBoot to not finish certain requests and causing Selenium-webdriver to throw Net::ReadTimeout errors.

I've been trying to create a minimal test case that reproduces this issue but not much luck yet.

I posted a Stack Overflow question: https://stackoverflow.com/questions/52632353/capybara-selenium-webdriver-netreadtimeout-when-using-rspec-file-fixtures-fa