Why:
* There were architectural issues with how the permit matcher kept track
of params instances on which doubles had been placed. Previously we
were starting off by taking the ActionController::Parameters class and
stubbing the #permit and #require instance method on it -- in other
words, we were stubbing #require for all instances of
ActionController::Parameters -- then we would stub #permit on a
particular instance of ActionController::Parameters that #require
returned. What this means is that if for some reason the #permit stub
on an individual instance isn't working properly, then the #permit
stub on ActionController::Parameters will respond to the invocation.
This is exactly what happened for the issue we recently fixed --
if the stubbing were done a different way we wouldn't have run into
that issue.
* Also, there's no reason to have both ParametersDoubles and
SliceOfParametersDoubles classes around. While it's nice that we have
a simpler option to use if we don't need the more complex one, we
actually don't need a distinction here, and we can afford one class
that does both.
To satisfy the above:
* When stubbing #permit or #require, always do so on an instance of
ActionController::Parameters and not the whole class. This way we know
exactly which methods are being doubled and it's easier to debug things
in the future.
* This means that we now stub ActionController::Parameters.new and then
place stubs on the returned instance.
* Refactor ParametersDoubles and SliceOfParametersDoubles: combine them
into a ParametersDoubleRegistry class, but extract the code that stubs
ActionController::Parameters.new into
a CompositeParametersDoubleRegistry class.
* Since this broke one of the tests, modify DoubleCollection so that a
method cannot be doubled more than once -- if the method is already
doubled then `register_stub` or `register_proxy` does nothing and
returns the original Double.
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