[JENKINS-26781] set $class in submitted form to use descriptor.id, so…

… descriptor can be retrieved by ID, which _may_ not be clazz.name

core/src/main/java/hudson/model/Descriptor.java

@@ -918,6 +918,7 @@ private URL getStaticHelpUrl(Klass<?> c, String suffix) {
                 if (d != null) {
                     items.add(d.newInstance(req, jo));
                 }
+                LOGGER.warning("Received unexpected formData for descriptor " + kind);
             }
         }
 

core/src/main/resources/lib/form/class-entry.jelly

@@ -34,7 +34,7 @@ THE SOFTWARE.
       structured form submission.  Mutually exclusive with clazz.
     </st:attribute>
   </st:documentation>
-  <j:set var="clazz" value="${attrs.clazz ?: attrs.descriptor.clazz.name}" />
+  <j:set var="clazz" value="${attrs.clazz ?: attrs.descriptor.id}" />
   <f:invisibleEntry>
     <!-- Legacy: Remove once plugins have been staged onto $class -->
     <input type="hidden" name="stapler-class" value="${clazz}" />