Mark HTTP requests as HTTP/1.1

* src/http.c (send_request): Use HTTP/1.1

As of now the PayPal servers responds to requests in HTTP 1.0 mode
with "Bad Request". However that does only happen with the sandbox
servers and not (yet) with the live servers. Although we _may_ not
fully support HTTP 1.1 we switch to 1.1 anyway.

• Signed-off-by: Werner Koch <wk@gnupg.org>