Fix "Scaling Rails Screencasts" link in caching guide

Closes #10241 [ci skip]
Use secure source in generated Gemfile for creating new plugin. Closes #10245
Merge pull request #10246 from prathamesh-sonpatki/3.2.13-https-rubygems

Using secure source https://rubygems.org when generating new plugin with Rails 3.2
adding a test for root path in the app
just clear the caches on clear! rather than replacing. fixes #10251
Merge branch '3-2-stable' of git://github.com/rails/rails into 3-2-stable