* **Fix bug.** If a cloudformation stack is updated with a new
parameter, that parameter should be honored. Several unit tests
had bugs where they were not providing parameters required by the template.
* **Fix bug.** Do not update stack parameters until after deleting removed
resources, so that any references to removed parameters can be resolved.
* **Fix bug.** Per the API, creation of a change set should not modify
a stack. The `diff` method, called in the creation of a
FakeChangeSet, was mutating the resource map which was problematic
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