Change the system tests to set Puma as default server only when the user haven't specified manually another server.

[guilleiguaran]

In our app we are configuring a custom server for Capybara in this way:

Capybara.register_server(:unicorn) do |app, port, host|
  Unicorn::HttpServer.new(app, listeners: "#{host}:#{port}", logger: Logger.new(nil)).start.join
end

Capybara.server = :unicorn

But System Tests are overriding the Capybara.server to :puma so our configuration isn't taking effect.

With this change, the Capybara.server is set only if the user hasn't customized it manually (so it's other than :default).

[eileencodes]

What do you think about adding Unicorn as a server option instead? I originally had support for that but since Puma was the default for rails we dropped support for setting drivers.

We can also consider doing that later. This PR works if you're in a bind. 😄

[guilleiguaran]

@eileencodes will do it!! but first I'll prepare a PR to add it as server option in Capybara side