Prevent Rails load hooks applying patch twice

[delner]

After refactoring the Rails integration in #552 and #540, we applied a new patching strategy that used ActiveSupport.on_load(:before_initialize) to run Rails patching code once at the appropriate time.

Turns out in some applications, these load hooks can run twice, which injects the middleware twice and causes bad side effects.

Although Rails 5.0+ offers a run_once option for these load hooks, they aren't available for Rails 3.0 - 4.2, which we currently support. Instead, I added a :for option to our existing do_once function which adds a second dimension to the patch hash, then used the Rails application instance as a key.

[soulcutter]

I tested this out in our app that had the "double middleware" problem, and this does indeed fix the problem! Thanks for digging into this!

My one small suggestion is pretty cosmetic; no worries if you take it or leave it.