Revert "Revert changes related to `bundle install` fixes in `rails new`"

The cause of the previous revert was bug in bundler that made it hard to
make railties test work. Fix for bundler was recently pushed to github,
so now we can safely get back to the original commit.

This reverts commit 0f5cc34ab5.

railties/lib/rails/generators/app_base.rb

@@ -246,8 +246,17 @@ module Rails
         # is easier to silence stdout in the existing test suite this way. The
         # end-user gets the bundler commands called anyway, so no big deal.
         #
+        # We unset temporary bundler variables to load proper bundler and Gemfile.
+        #
         # Thanks to James Tucker for the Gem tricks involved in this call.
-        print `"#{Gem.ruby}" "#{Gem.bin_path('bundler', 'bundle')}" #{command}`
+        _bundle_command = Gem.bin_path('bundler', 'bundle')
+
+        bundle_bin_path, bundle_gemfile, rubyopt = ENV['BUNDLE_BIN_PATH'], ENV['BUNDLE_GEMFILE'], ENV['RUBYOPT']
+        ENV['BUNDLE_BIN_PATH'], ENV['BUNDLE_GEMFILE'], ENV['RUBYOPT'] = "", "", ""
+
+        print `"#{Gem.ruby}" "#{_bundle_command}" #{command}`
+
+        ENV['BUNDLE_BIN_PATH'], ENV['BUNDLE_GEMFILE'], ENV['RUBYOPT'] = bundle_bin_path, bundle_gemfile, rubyopt
       end
 
       def run_bundle

railties/test/generators/plugin_new_generator_test.rb

@@ -115,7 +115,13 @@ class PluginNewGeneratorTest < Rails::Generators::TestCase
   end
 
   def test_generation_runs_bundle_install_with_full_and_mountable
-    result = run_generator [destination_root, "--mountable", "--full"]
+    result = run_generator [destination_root, "--mountable", "--full", "--dev"]
+    assert_file "#{destination_root}/Gemfile.lock" do |contents|
+      assert_match(/bukkits/, contents)
+    end
+    assert_match(/run  bundle install/, result)
+    assert_match(/Using bukkits \(0\.0\.1\)/, result)
+    assert_match(/Your bundle is complete/, result)
     assert_equal 1, result.scan("Your bundle is complete").size
   end
 
@@ -394,4 +400,3 @@ protected
     silence(:stdout){ generator.send(*args, &block) }
   end
 end
-