Disallow "#" as an $id.

In a document root schema, "$id": "#" is a no-op, and in
an embedded schema resource root, it results in the embeddded
resource having the same URI as its context resource.  Given
that the specificaiton says implementations SHOULD raise an error
for duplicate IRIs, there is no reason to allow this form.

Author: handrews

jsonschema-core.xml

@@ -1353,8 +1353,8 @@
                     </t>
                     <t>
                         If present, the value for this keyword MUST be a string, and MUST represent a
-                        valid <xref target="RFC3987">IRI-reference</xref>.  This IRI-reference
-                        SHOULD be normalized, and MUST resolve to an
+                        valid <xref target="RFC3987">IRI-reference</xref> with a non-empty non-fragment
+                        component.  This IRI-reference SHOULD be normalized, and MUST resolve to an
                         <xref target="RFC3987">absolute-IRI</xref> (without a fragment),
                         or to a IRI with an empty fragment.
                     </t>