Fix broken button captions in listings

[MAILPOET-1676]

assets/js/src/listing/bulk_actions.jsx

@@ -110,7 +110,7 @@ class ListingBulkActions extends React.Component {
         <input
           onClick={this.handleApplyAction}
           type="submit"
-          defaultValue={MailPoet.I18n.t('apply')}
+          value={MailPoet.I18n.t('apply')}
           className="button action"
         />
 

assets/js/src/listing/filters.jsx

@@ -67,7 +67,7 @@ class ListingFilters extends React.Component {
           id="post-query-submit"
           onClick={this.handleFilterAction}
           type="submit"
-          defaultValue={MailPoet.I18n.t('filter')}
+          value={MailPoet.I18n.t('filter')}
           className="button"
         />
       );