apm.Context.SetHTTPRequest wipes c.user.Username set manually

[mcgrawia]

Describe the bug
In my application, I am manually calling apm.Context.SetUsername to add user info to the transaction. When the transaction is set to the APM server, the username is wiped from the data in SetHTTPRequest. I've traced the problem to the following lines, which don't take into account when c.user.Username is already set:
https://github.com/elastic/apm-agent-go/blob/master/context.go#L206-L213

	username, _, ok := req.BasicAuth()
	if !ok && req.URL.User != nil {
		username = req.URL.User.Username()
	}
	c.user.Username = truncateString(username)
	if c.user.Username != "" {
		c.model.User = &c.user
	}

It seems like this line: c.user.Username = truncateString(username) sets both c.user.Username and c.model.User.Username since they point to the same model.User object.

To Reproduce

1. In your http application, call tx.Context.SetUsername(username)
2. observe no username is present in the APM events

Expected behavior
Username should be set in the APM events.

[axw]

Thanks for the report @mcgrawia! I'll get onto a fix shortly.

[mcgrawia]

thanks for the quick fix @axw! the change works great.