warn when the app passed doesnt respond to #call

lib/capybara/session.rb

@@ -67,6 +67,7 @@ module Capybara
     attr_accessor :synchronized
 
     def initialize(mode, app=nil)
+      raise ArgumentError, "The second parameter to Session::new should be a rack app if passed." if app && !app.respond_to?(:call)
       @mode = mode
       @app = app
       if Capybara.run_server and @app and driver.needs_server?

spec/session_spec.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+require 'spec_helper'
+
+RSpec.describe Capybara::Session do
+  it "verifies a passed app is a rack app" do
+    expect do
+      Capybara::Session.new(:unknown, { random: "hash"})
+    end.to raise_error ArgumentError, "The second parameter to Session::new should be a rack app if passed."
+  end
+end