[couchbase2] fix field returning on decode.

this properly fixes #786 which has been reverted since it has some
side effects on other workloads and causes regressions.

couchbase2/src/main/java/com/yahoo/ycsb/db/couchbase2/Couchbase2Client.java

@@ -856,7 +856,7 @@ private void decode(final String source, final Set<String> fields,
       for (Iterator<Map.Entry<String, JsonNode>> jsonFields = json.fields(); jsonFields.hasNext();) {
         Map.Entry<String, JsonNode> jsonField = jsonFields.next();
         String name = jsonField.getKey();
-        if (checkFields && fields.contains(name)) {
+        if (checkFields && !fields.contains(name)) {
           continue;
         }
         JsonNode jsonValue = jsonField.getValue();