Modify KafkaSchemaRecordType.

Author: Bingqin Zhou

kcbq-connector/src/main/java/com/wepay/kafka/connect/bigquery/convert/KafkaSchemaRecordType.java

@@ -5,12 +5,16 @@
  *  Enum class for Kafka schema or record type, either value or key.
  */
 public enum KafkaSchemaRecordType {
+
     VALUE("value"),
     KEY("key");
-    String str;
+
+    private final String str;
+
     KafkaSchemaRecordType(String str) {
         this.str = str;
     }
+
     public String toString() {
         return this.str;
     }