Add error handling for missing server_url in new_http_client

Author: rgisiger

rb/lib/selenium/webdriver/remote/http/default.rb

@@ -119,6 +119,8 @@ def response_for(request)
           end
 
           def new_http_client
+            raise Error::WebDriverError, 'server_url not set' unless server_url
+
             if proxy
               url = proxy.http
               unless proxy.respond_to?(:http) && url