update rails_template.rb for rails 4.x

remove the "public/index.html" file only if it exist, rails 4.x don't have this file.
cosmetic
implement missing filter form builder spec #1
implement missing filter form builder spec #2
Merge pull request #3357 from timoschilling/specs2

add missing filter form builder specs
Merge pull request #3353 from timoschilling/fix_rails_template

update rails_template.rb for rails 4.x