Implement ActionController::Parameters#inspect

[bquorning]

Now that AC::Parameters is no longer a Hash, it shouldn't look like a Hash.

Here's an example from my RSpec output without this PR:

#<Foo:0x007f10be4aa4b8> received :bar with unexpected arguments
  expected: ({"x"=>"y"})
       got: ({"x"=>"y"})
Diff:

Please stub a default value first if message might be received with other args as well.

After this PR, the diff is much more useful:

#<Foo:0x007f5dc8799848> received :bar with unexpected arguments
  expected: ({"x"=>"y"})
       got: (<ActionController::Parameters {"x"=>"y"}>)
Diff:
@@ -1,2 +1,2 @@
-[{"x"=>"y"}]
+[<ActionController::Parameters {"x"=>"y"}>]

Please stub a default value first if message might be received with other args as well.

[rails-bot]

r? @pixeltrix

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[senny]

@bquorning thank you 💛