v0.5.1

• Media type normalization and matching moved to a shared MediaType helper.
  The validators and the multipart decoder each carried their own copy of the
  wildcard rules, so */*, type/* and *+json were decided in two places
  that could drift; the multipart copy also skipped the normalization the
  others did, and now folds parameters and case off a part's declared type as
  well.
• The properties reader the form and multipart decoders duplicated verbatim
  is one method on SchemaValueDecoder.
• Whether an undecodable body can be judged against its schema is one
  fail-closed decision, OpaqueBodyVerdict, that the request and response
  validators map to their own wording. The two directions carried separate
  copies of the rule.
• RequestValidator computes the constraining schema once per body instead of
  twice.
• Filtering every property of an object for a direction leaves the document's
  own openness intact — additionalProperties: false still closes it, and its
  absence still leaves it open. Behavior is unchanged; it is now pinned by a
  test, because the open half reads like a hole in a fail-closed package and
  is not one.