Use default HTTP client when communicating with services

This allows to share a logic for setting default headers (e.g User-Agent)
and any additional processing required to make HTTP requests.

Closes #6662

rb/lib/selenium/webdriver/common/service.rb

@@ -135,12 +135,12 @@ def build_process(*command)
       end
 
       def connect_to_server
-        Net::HTTP.start(@host, @port) do |http|
-          http.open_timeout = STOP_TIMEOUT / 2
-          http.read_timeout = STOP_TIMEOUT / 2
-
-          yield http
-        end
+        http = Selenium::WebDriver::Remote::Http::Default.new
+        http.open_timeout = STOP_TIMEOUT / 2
+        http.read_timeout = STOP_TIMEOUT / 2
+        http.server_url = uri
+        yield http
+        http.close
       end
 
       def find_free_port
@@ -164,7 +164,7 @@ def stop_process
       def stop_server
         return if process_exited?
 
-        connect_to_server { |http| http.get('/shutdown') }
+        connect_to_server { |http| http.call(:get, '/shutdown', nil) }
       end
 
       def process_running?