Rewrite tests for ensure_inclusion_of

ensure_inclusion_of is pretty complex. Not only does it support two ways
of specifying valid values (in_array, in_range), but it also supports
multiple column types (string, integer, float, decimal, boolean).

When it comes to handling numbers and strings, ensure_inclusion_of has
very similar logic, and we can test these cases similarly.

We also do not have tests for handling regular Ruby attributes as
opposed to database columns (validates_inclusion_of works with both).
Those tests will be added in a future commit, this commit lays the
groundwork for that.
Add a small stubbing library

This provides a robust solution for temporarily stubbing (and
unstubbing) methods. It will be internally by the strong parameters and
delegation matchers.
Fix several issues with strong parameters matcher

* Instead of decorating controller params, use our "Doublespeak"
  mini-library to stub ActionController::Parameters. This prevents from
  interfering with the params object if it is used in various ways, i.e.
  if `params.fetch(...).permit(...)` is used instead of
  `params.require(...).permit(...)`.
* Fix compat with Rails 4.1, where the verb for #update is PATCH not
  PUT
* Track multiple calls to #permit within a given controller action
* Fix so that if the route for your action requires params (such as
  :id) then you can specify those params
Fix delegate_method to double objects properly

Use our Doublespeak mini-library to stub the target method on the object
in question, ensuring that the stubs we create during matching get
reversed when the match ends.

Also:

* Make it possible to negate the matcher, and use negation in tests
  rather than !matches?
* If the matcher fails, add more information to the failure message as
  to what was actually called on the target object
Fix ensure_inclusion_of to support plain attrs

Also add missing tests to confirm this behavior.
Fix NonNullableBooleanError so it contains attribute name
Bump to version 2.6.1.rc1