DiffFieldReader: filter all '#' fields from sets

Now that readMap filters out '#' fields, when maps are nested in sets,
we exposed a related bug where a set was iterating over nested maps and
expected the '#' key to be present in those nested maps.

By skipping _all_ count fields when iterating over set keys, all is
right with the world again.

helper/schema/field_reader_diff.go

@@ -153,8 +153,8 @@ func (r *DiffFieldReader) readSet(
 		if !strings.HasPrefix(k, prefix) {
 			continue
 		}
-		if strings.HasPrefix(k, prefix+"#") {
-			// Ignore the count field
+		if strings.HasSuffix(k, "#") {
+			// Ignore any count field
 			continue
 		}