Username bug when calling predict

Currently in the `predict` method the logic is as follows:

```
prediction_url = urllib.parse.urljoin(self.url, os.path.join(
self.username, 'models', modelName, 'predict'))
username = username if username else self.username
response = requests.post(
url=prediction_url,
...
```
The way this logic works is that it builds the prediction url with `self.username` always and never considers if you give the `predict` method a `username` along with the `model` and `data`. It should be switched to look like this:

```
username = username if username else self.username
prediction_url = urllib.parse.urljoin(self.url, os.path.join(
username, 'models', modelName, 'predict'))
```
Unfortunately the repo is read-only so I can not push the change to a remote branch and open a PR. In the short-term I have modified my source code to work as intended.