MDL-52214 core: Fix case sensitivity in user agent comparison

The previous 2.9 implementation of is_web_crawler() used stripos() in
certain cases. The unit tests re-added in the previous commit revealed
that certain crawlers (such as BaiDuSpider) were not correctly detected
in the new refactored implementation.

It seems lesser evil and safe enough to use /i in the regex search even
though it is not 100% same logic as before - as stripos() was used in
some cases only, not always.

lib/classes/useragent.php

@@ -215,7 +215,7 @@ class core_useragent {
      * @return bool
      */
     protected function is_useragent_web_crawler() {
-        $regex = '/Googlebot|google\.com|Yahoo! Slurp|\[ZSEBOT\]|msnbot|bingbot|BingPreview|Yandex|AltaVista|Baiduspider|Teoma/';
+        $regex = '/Googlebot|google\.com|Yahoo! Slurp|\[ZSEBOT\]|msnbot|bingbot|BingPreview|Yandex|AltaVista|Baiduspider|Teoma/i';
         return (preg_match($regex, $this->useragent));
     }