Only merge params option if params is a Hash in url_for helper

[ghost]

This PR Fixes the problem described in this original issue that was fixed here and reverted here

The fix was reverted because the keyword params is special and is used to make possible to pass parameters that are reserved.

So this fix suggests to only merge params when it is a Hash.

The example in the revert commit from @rafaelfranca suggests that params, when used, takes the form of a Hash.

closes #42028
closes #42052

[benoittgt]

Don't we need a test?

[sholden]

@benoittgt I added a test in the pull request @marivaldo just referenced, can pluck it from there. It actually tests the issue through a controller test case, which is how I first encountered the bug, but it might be better to test more directly.

[ghost]

@benoittgt I made a test for when params are not a hash. which made me realise that url_for is overriding existing params in this line.
In other words, if a special param called params is being passed, it will get deleted (or replaced). Therefore, in that special case, we want to keep the param called params in params[:params]

if you inspect the options that url_for built in that test case, you'll get something like
{:only_path=>true, :controller=>"c", :action=>"a", :params=>{:params=>"p"}, :path=>"/c/a", :script_name=>"", :user=>nil, :password=>nil}

[zenspider]

nudge

[casperisfine]

So this broke our app quite extensively as it's not rare for options[:params] to be an ActionController::Parameters.

I'll fix it in #42603, feel free to chime in.