Expose Net::HTTP#keep_alive_timeout= as :keep_alive_timeout .

I propose exposing `Net::HTTP#keep_alive_timeout=`, to join existing `Net::HTTP#read_timeout=` and `Net::HTTP#open_timeout=` exposure. This could assist with certain types of connection configurations, particularly when using RestClient to connect to an API using a long-living connection.

Peace,
tiredpixel

---

Similar to existing `:timeout` (`Net::HTTP#read_timeout=`),
`:open_timeout` (`Net::HTTP#open_timeout=`), add `:keep_alive_timeout`
(`Net::HTTP#keep_alive_timeout`).

Note that timeout `-1` => `nil` with warning is replicated for
consistency with `:timeout`,`:open_timeout`, despite
`:keep_alive_timeout` being new.