Allow pattern attribute in input

"pattern" included in the list of allowed attributes in the input tag

program/lib/Roundcube/html.php

@@ -393,7 +393,7 @@ class html_inputfield extends html
         'type','name','value','size','tabindex','autocapitalize','required',
         'autocomplete','checked','onchange','onclick','disabled','readonly',
         'spellcheck','results','maxlength','src','multiple','accept',
-        'placeholder','autofocus',
+        'placeholder','autofocus','pattern'
     );
 
     /**