Use Rack to generate query information under test

[tenderlove]

to_query sorts parameters before encoding them. This causes a round
tripping issue as noted here:

#23997 (comment)
#10529 (comment)
#30558

Unfortunately, that method is being used to generate cache keys, so its
results need to be stable:

10dec0e

However, the test harness is only using to_query to encode parameters
before sending them to the controller so the "cache key" usecase doesn't
apply here.

This commit adds a test that demonstrates the round trip problems and
changes the serialization strategy to use Rack for encoding the
parameters rather than to_query.

[tenderlove]

I think the GET case has the same problem, but I don't have a failing test for it yet so I didn't touch it.

[rafaelfranca]

This cause a regression when you pass Active Record objects to the parameters. Before when you do:

post :foo, params: { user: User.first }

You would get in the controller { "user" => "1" }.

Now you will get { "user": "#<User: ...>" }.

This change in behavior should be fine for the master branch if we deprecate it, but for stable branches this will break some people tests. (I found this because some tests broke when upgrading shopify to the stable branch)