data_loader.ex does not customize default hackney User-Agent string

[ptudor]

Hi,

It looks like the data_loader.ex file does not customize the default user agent. It would be useful to set that to a unique value to politely identify the source of http requests.

Untested code would be something like...
Before:

{:ok, 200, _headers, client_ref}=:hackney.get(url, [], "", [])

After:

tz_ua = [<<"User-Agent: ", "tzdata/", version/binary, " (https://github.com/lau/tzdata)" >>],
{:ok, 200, _headers, client_ref}=:hackney.get(url, tz_ua, "", [])

For the sake of reference easily copy-and-pasted, hackney's default_ua() is created in hackney_request.erl.

Thanks,