Fix mobile specs

These were (a) trying to use a non-existent session, and (b) trying to
set both `mobile: true` and a resolution, which is now disallowed.

Author: duncan-bayne

lib/capybara/cuprite/page.rb

@@ -144,7 +144,11 @@ def prepare_page
         super
 
         width, height = @options.window_size
-        resize(width: width, height: height, mobile: @options.mobile)
+        if @options.mobile
+          resize(width: 0, height: 0, mobile: @options.mobile)
+        else
+          resize(width: width, height: height, mobile: @options.mobile)
+        end
 
         if @options.url_blacklist.any?
           network.blacklist = @options.url_blacklist

spec/spec_helper.rb

@@ -40,6 +40,7 @@
 
 module TestSessions
   Cuprite = Capybara::Session.new(:cuprite, TestApp)
+  CupriteMobile = Capybara::Session.new(:cuprite_mobile, TestApp)
 end
 
 RSpec.configure do |config|