Allow multiple StrongParametersMatchers to exist at once.

The provided RSpec example looks a bit contrived, but it corresponds to
this perfectly reasonable Minitest test case that was not working:

  class UserControllerTest < ActionController::TestCase
    should permit(:name).for(:create)
    should_not permit(:admin).for(:create)
  end

This instantiates two matchers at class load time, which resulted in the
second one overriding the double collection of the first one. Then when
the tests are executed, the first matcher would fail, because its double
is not listening to ActionController::Parameters#permit -- only the
second matcher's double collection gets activated.

This is fixed in Doublespeak by only creating one double collection per
class.