Set connection.keep_alive_timeout using idle_timeout in ruby 2.0.

Ruby 2.0 added keep_alive_timeout, which behaves the same way as
idle_timeout, except that the default timeout is 5 seconds rather than 2
seconds. Set keep_alive_timeout using idle_timeout to prevent the
connection from getting closed before idle_timeout due to delays between
requests.

lib/net/http/persistent.rb

@@ -631,6 +631,7 @@ def connection_for uri
     start connection unless connection.started?
 
     connection.read_timeout = @read_timeout if @read_timeout
+    connection.keep_alive_timeout = @idle_timeout if @idle_timeout && connection.respond_to?(:keep_alive_timeout=)
 
     connection
   rescue Errno::ECONNREFUSED

test/test_net_http_persistent.rb

@@ -274,6 +274,7 @@ def test_certificate_equals
   def test_connection_for
     @http.open_timeout = 123
     @http.read_timeout = 321
+    @http.idle_timeout = 42
     c = @http.connection_for @uri
 
     assert_kind_of @http_class, c
@@ -283,6 +284,7 @@ def test_connection_for
 
     assert_equal 123, c.open_timeout
     assert_equal 321, c.read_timeout
+    assert_equal 42, c.keep_alive_timeout unless RUBY_1
 
     assert_includes conns[0].keys, 'example.com:80'
     assert_same c, conns[0]['example.com:80']