code: public String invokeUpdateHandler(String updateHandlerUri, String docId,
                                        Params params) {
          assertNotEmpty(params, "params");
          return db.invokeUpdateHandler(updateHandlerUri, docId, params.getInternalParams());
      }
docs: Invokes an Update Handler.
  <P>Example usage:</P>
  <pre>
  {@code
  final String newValue = "foo bar";
  Params params = new Params()
  .addParam("field", "title")
  .addParam("value", newValue);
  String output = db.invokeUpdateHandler("example/example_update", "exampleId", params);
  
  }
  </pre>
  <pre>
  Params params = new Params()
  .addParam("field", "foo")
  .addParam("value", "bar");
  String output = dbClient.invokeUpdateHandler("designDoc/update1", "docId", params);
  </pre>
  
  @param updateHandlerUri The Update Handler URI, in the format: <code>designDoc/update1</code>
  @param docId            The document id to update.
  If no id is provided, then a document will be created.
  @param params           The query parameters as {@link Params}.
  @return The output of the request.
  @see <a
  href="https://console.bluemix.net/docs/services/Cloudant/api/design_documents.html#update-handlers"
  target="_blank">Design documents - update handlers</a>