Include a User-Agent header in import calls.

[tumes]

Cloudflare BIC requires a user agent for incoming calls. As such, if one is running a wordpress blog and that blog is using the Cloudflare plugin or otherwise has Cloudflare in front of it, chances are very likely any attempt to import will fail both for initial probe and the actual import process. Consequently the import pipeline is severely limited for a very common use case. Should be just a few lines of change.