Get environment from RACK_ENV if exists by default

Hi, it may be a good idea to get the environment from RACK_ENV by default. 

We use this in production and recently traced a bug back to the environment not being set in Guard::Puma, so I figured others might have made the same mistake.
I'm always a bit paranoid about setting and deleting env vars. I doubt it would ever effect someone's machine, but I think it's a good practice to reset here by storing the value and setting to something specific.

```ruby
before do
  @rack_env = ENV['RACK_ENV']
end

# existing spec, minus the call to `ENV.delete`

after do
  ENV['RACK_ENV'] = @rack_env
end


```