Allow "headerSchema" for content negotiation

While reworking the spec for overall flow, it became apparent
that a valuable use of "headerSchema" is to advertise content
negotiation options.  However, as currently worded, this usage
is forbidden.

Change the usage to allow this without being too burdensome
on implementations.

The rewrite will provide greater clarity on the general
implementation requirements of "targetHints" and "headerSchema",
so please do not worry too much about that for now.

Author: handrews

jsonschema-hyperschema.xml

@@ -1235,9 +1235,18 @@ GET /foo/
                 </t>
 
                 <t>
-                    If this property's value is not specified, then the value should be taken to be
-                    "application/json".  Hyper-Schema authors SHOULD NOT use a protocol-specific
-                    value in <xref target="targetHints">"targetHints"</xref> for this purpose.
+                    For protocols supporting content-negotiation, implementations MAY choose to
+                    describe possible target media types using protocol-specific information in
+                    <xref target="headerSchema">"headerSchema"</xref>.  If both protocol-specific
+                    information and "mediaType" are present, then the value of "mediaType" MUST
+                    be compatible with the protocol-specific information, and SHOULD indicate
+                    the media type that will be used in the absence of content negotiation.
+
+                </t>
+                <t>
+                    When no such protocol-specific information is available, or when the
+                    implementation does not recognize the protocol involved, then the value
+                    SHOULD be taken to be "application/json".
                 </t>
 
                 <figure>