Vendor'd rails gems can't be "depended"

If a gem depends on a rails gem, e.g., activesupport, if the vendor'd version is not also in the system repository, an error occurs.

For example, right now I have 2-1-stable vendor'd, which reports its gem version as 2.1.2. If I config.gem thoughtbot-factory_girl, script/console errors out with

/usr/lib64/ruby/site_ruby/1.8/rubygems.rb:150:in `activate':Gem::Exception: can't activate activesupport (>= 1.0, runtime), already activated activesupport-2.1.2

In initalizer.rb, rails bashes gem's Gem.loaded_specs[stub] so that it knows activesupport is loaded. However, when trying to do the version matching, this isn't enough to get gem to consider the vendor'd rails version as possibility.

Changing

stubs.each do |stub|
  Gem.loaded_specs[stub] = Gem::Specification.new do |s|
    s.name = stub
    s.version = Rails::VERSION::STRING
  end
end

to

stubs.each do |stub|
  Gem.loaded_specs[stub] = Gem::Specification.new do |s|
    s.name = stub
    s.version = Rails::VERSION::STRING
  end
  Gem.source_index.add_spec(Gem.loaded_specs[stub])
end

makes the vendor'd version a valid candidate and fixes the problem. This doesn't force the vendor'd version to be okay: it's still checked against the requiring gem's spec and will still error out if the vendor'd version doesn't meet the requirements.