User Agent is not set for every HTTP request

Application firewalls tend to block HTTP requests without any User Agent set. Some of the curl requests set the User Agent, but not all do. The following don't:

• phpCAS in auth/cas (created a pull request) merged but not released yet
• lib\adminlib.php (here)
• lib\behat\classes\util.php (here)
• lib\editor\tinymce\plugins\spellchecker\classes\GoogleSpell.php (here)
• lib\ltiprovider\src\HTTPMessage.php (here)
• lib\testing\lib.php (here)

I would be happy to create a pull request to include a User Agent in these locations. This affects older versions of Moodle, so I could also create fixes for older version as well. Should I follow filelib in using the User Agent defined by `
core_useragent::get_moodlebot_useragent()`
This issue was preventing my instance of Moodle from working, but I created this as an improvement rather than a bug given that an external change caused Moodle not to work.