Update default User-Agent.
Update default User-Agent.

Updated user agent to reflect current version.
This should be build from version constant, that should be
automatically updated with releases, not sure how to do it yet.

canassa/http.async.client,diamondap/http.async.client,bpoweski/http.async.client,josephwilk/http.async.client,oliyh/http.async.client,michaelneale/http.async.client