We previously sent our client type to the server as a GET/POST parameter
of "client=<something>", most commonly "client=API: Python".
We switch here to providing the same information as a User-agent header
sent on each request, which is more standards-compliant.
Also added is some data about the platform the user is using. If your
client string was set to "MyLittleZulip/1.0", the resultant string could
look something like this:
MyLittleZulip/1.0 (Ubuntu; 12.04)
(imported from commit 39fd187a8f9d4b3c9b63fc623e0836e57a4099ca)