Gem tasks fail when cache_classes=true

If I have a model like:

# MyModel.rb
require 'fastercsv'
class MyModel < ActiveRecord::Base; end

and in my config file I have

# ....
config.cache_classes = true
config.gem 'fastercsv'

and I don't have the fastercsv gem installed, the rake gems tasks cannot run.

In initializer.rb, cache_classes=true ends up kicking off the class preloading, which then ends up trying to require a gem that's not yet been installed, ... (another gems:install catch-22).

The easy fix is fairly straightforward (will provide a patch & tests), but load_application_classes needs to just check for $rails_gem_installer to be set, and if so don't try and do the preloading.

It kind of seems like the $rails_gem_installer flag is a bit of a hack, and that really rake gems:install should be setting what initializer steps get ran (seems like very few of them), but that's a bigger fix.

Here's a workaround you can drop into environment.rb in the mean-time

# this is a workaround for a Rails issue where cache_classing can lead to not yet installed
# gems to cause the gem:install to fail
module Rails
  class Initializer
    def load_application_classes_with_check_for_gem_loading
      load_application_classes_without_check_for_gem_loading unless $rails_gem_installer
    end
    alias_method :load_application_classes_without_check_for_gem_loading, :load_application_classes
    alias_method :load_application_classes, :load_application_classes_with_check_for_gem_loading
  end
end