ServiceTask extensionElements are removed at Process Definition edit

**Describe the bug**
When a process definition file having an _extensionElements_ element within a _serviceTask_ element is edited, _extensionElements_ are removed from the XML.
For example, when you edit a Process Definition with a serviceTask having a _failedJobRetryTimeCycle_ setting like bellow:
```xml
R60/PT1M
```
The corresponding xml file will just contain the following:
```xml

```

**Expected behavior**
_extensionElements_ should not be dropped from process definition as this is an official Flowable feature. See documentation here: https://flowable.org/docs/userguide/index.html#failRetry