Can't set referrer url in ServerEvent

[marinrusu1997]

Which SDK version are you using?

v19.0.3

What's the issue?

The ServerEvent doesn't fully implement documentation, namely it doesn't contain referrer_url.

Steps/Sample code to reproduce the issue

Observed Results:

• referrer_url can't be set and sent through this current version of sdk

Expected Results:

• We should be able to set referrer_url through SDK

[bendalgarn]

This remains an issue in v20.0.2.

[jonasatmonday]

We need this functionality as well (but in the Java SDK). Pushing it here!