Resolve $refs nested inside parameter and response header schemas

[ahx]

Fixes ahx/openapi_first#450.

Builder handed Parameters::Parameter (and Header#resolved_schema) a schema that was resolved
only one level deep: RefResolver::Hash#resolved follows a $ref at the top level of a node, but
every $ref below that stayed unresolved. The parameter unpacking and conversion code reads those
plain hashes, so it saw no type, items or properties behind a nested $ref and left the value
alone, while json_schemer — which does follow refs — then rejected it.

So besides the reported deepObject case, values described via items: { $ref: … },
allOf: [{ $ref: … }] or a $ref'd object property were not converted either, for query, path,
header and cookie parameters as well as response headers.

RefResolver::Hash#dereferenced / Array#dereferenced now return a plain, fully dereferenced
Hash/Array, and Builder uses it for parameter and response header schemas. This happens once per
definition, so there is no cost per request, and validation itself is untouched — only unpacking and
type conversion gain the type information they were missing. A $ref that points back to a node on
its own path (a recursive schema) is left unresolved, so loading such a document still terminates
and the result stays JSON-serializable.

Also in here, from the original draft: request examples in spec/test_cases_spec.rb
(valid_request / invalid_request), a few nullable request body cases and a readable
Failure#inspect.