In Apipie#865 we introduced a `method_name` method on `MethodDescription` to avoid this bug, but the commit didn't actually use that method.  Sometimes the `@controller_method` object used is a `Apipie::Generator::Swagger::MethodDescription::Decorator` which is a `SimpleDelegate` onto a `Apipie::MethodDescription` and we'd expect to be able to call `method` on it, but unfortunately `method` is one of the things _not_ delegated by `SimpleDelegate` because it's a standard ruby method and so you get

    ArgumentError: wrong number of arguments (given 0, expected 1)

when you try to call it.  Using `method_name` instead avoids that so that's what we do - and now we can happily generate the swagger warnings when we have hash type objects without defined params.

Unlike the previous commit, this isn't strictly required as we're not calling
the un-delegated `method`, but instead the warning will contain a standard
`to_s` of the `@controller_method` that isn't very easy to read, like this:

    WARNING (105): [#<Apipie::MethodDescription:0x0000000126316f60>] -- The
    parameter :param is optional but default value is not specified (use
    :default_value => ...)

By using `method_name` instead we get symmetry with the hash warning from the
previous commit.

Only allowed 15 `let` or `subject`s for a given spec, and we now have 16.
Removing the `with_null` memoization lets us proceed. In theory it would have
allowed us to run the specs with_null set to both true and false, but in
practice, we weren't, and the other memoized values _were_ useful for
customising the specs.

If we're generating swagger via
`SwaggerGenerator.json_schema_for_self_describing_class` we explicitly don't
have a `controller_method` being passed around so we can't use it for the
warnings. Introduce a test for type and builder to cover this scenario (first
spotted by a failing spec for `SwaggerGenerator`) and then change the
implementation to cope with it. Luckily,
`Apipie::Generator::Swagger::MethodDescription::Decorator` already had a
`ruby_name` implementation that handles being given `nil`, so we use that.