Allow "schema" media type parameter for schemas

application/schema+json MAY use the same mechanism as
application/schema-instance+json to identify its meta-schema,
although with application/schema+json, the "$schema" value
within the document is considered the canonical meta-schema.

Author: handrews

jsonschema-core.xml

@@ -2138,6 +2138,14 @@ Link: <http://example.com/my-hyper-schema#>; rel="describedby"
                     When using the media type application/schema-instance+json, the "schema"
                     parameter MUST be supplied.
                 </t>
+                <t>
+                    When using the media type application/schema+json, the "schema" parameter
+                    MAY be supplied, If supplied, it SHOULD contain the same URI as identified
+                    by the "$schema" keyword, and MAY contain additional URIs.  The "$schema"
+                    URI MUST be considered the schema's canonical meta-schema, regardless
+                    of the presence of alternative or additional meta-schemas as a media type
+                    parameter.
+                </t>
                 <t>
                     The schema URI is opaque and SHOULD NOT automatically be dereferenced.
                     If the implementation does not understand the semantics of the provided schema,
@@ -2272,7 +2280,21 @@ User-Agent: product-name/5.4.1 so-cool-json-schema/1.0.2 curl/7.43.0
                     <list>
                         <t>Type name: application</t>
                         <t>Subtype name: schema+json</t>
-                        <t>Required parameters: N/A</t>
+                        <t>>Required parameters: N/A</t>
+                        <t>
+                            Optional parameters:
+                            <list style="hanging">
+                                <t hangText="schema:">
+                                    A non-empty list of space-separated URIs, each identifying
+                                    a JSON Schema resource.  The instance SHOULD successfully
+                                    validate against at least one of these meta-schemas.
+                                    Non-validating meta-schemas MAY be included for purposes such
+                                    as allowing clients to make use of older versions of
+                                    a meta-schema as long as the runtime instance validates
+                                    against that older version.
+                                </t>
+                            </list>
+                        </t>
                         <t>
                             Encoding considerations: Encoding considerations are
                             identical to those specified for the "application/json"