Remove respond_to? check for Ruby 2.1+ feature

Author: drbrain

lib/net/http/persistent.rb

@@ -621,7 +621,7 @@ def connection_for uri
     end
 
     http.read_timeout = @read_timeout if @read_timeout
-    http.keep_alive_timeout = @idle_timeout if @idle_timeout && http.respond_to?(:keep_alive_timeout=)
+    http.keep_alive_timeout = @idle_timeout if @idle_timeout
 
     return yield connection
   rescue Errno::ECONNREFUSED