Fix compile error in JsonSerDesHandlerTest

[gkomlossi]

> Task :schema-registry:schema-registry-serdes:compileTestJava FAILED
/registry/schema-registry/schema-registry-serdes/src/test/java/com/hortonworks/registries/schemaregistry/serdes/json/JsonSerDesHandlerTest.java:91: error: no suitable method found for toString(Charset)
      String serializedAsString = outputStream.toString(StandardCharsets.UTF_8);
                                              ^
    method ByteArrayOutputStream.toString(String) is not applicable
      (argument mismatch; Charset cannot be converted to String)
    method ByteArrayOutputStream.toString(int) is not applicable
      (argument mismatch; Charset cannot be converted to int)