Increase connection keep alive timeout to 30 seconds (#836)

One thing I forgot to look into was how long Ruby will hold a connection
open by default. It turns out that the language default is very low at
only two seconds. Here we increase it to 30 seconds, which is a more
reasonable default. I took this number from Go's `DefaultTransport`,
which seems to have been working pretty well so far.

I tested with a script that keeps a connection idle to Stripe for a long
period of time before issuing a new request and everything seems to be
working well.

lib/stripe/connection_manager.rb

@@ -40,8 +40,6 @@ def connection_for(uri)
 
       if connection.nil?
         connection = create_connection(u)
-
-        # TODO: what happens after TTL?
         connection.start
 
         @active_connections[[u.host, u.port]] = connection
@@ -92,6 +90,15 @@ def execute_request(method, uri, body: nil, headers: nil, query: nil)
                                  proxy_host, proxy_port,
                                  proxy_user, proxy_pass)
 
+      # Time in seconds within which Net::HTTP will try to reuse an already
+      # open connection when issuing a new operation. Outside this window, Ruby
+      # will transparently close and re-open the connection without trying to
+      # reuse it.
+      #
+      # Ruby's default of 2 seconds is almost certainly too short. Here I've
+      # reused Go's default for `DefaultTransport`.
+      connection.keep_alive_timeout = 30
+
       connection.open_timeout = Stripe.open_timeout
       connection.read_timeout = Stripe.read_timeout