Default `User-Agent` header could cause uninteded consequences

This is more of an informative message than a bug. The default user agent is configured in `/settings#fetching` and is configurable. Although, some sites can behave differently when a browser user agent is supplied.

```
; curl 'https://jira.atlassian.com/rest/issueNav/1/issueTable' -H 'X-Atlassian-Token: no-check' -X POST -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.66 Safari/537.36' -w '%{http_code}\n'
XSRF check failed404

; curl 'https://jira.atlassian.com/rest/issueNav/1/issueTable' -H 'X-Atlassian-Token: no-check' -X POST -H 'User-Agent: curl' -w '%{http_code}\n'
400
```

Found this the hard way.