44024: search form bootstrap 4 beta fix

src/app/shared/search-form/search-form.component.html

@@ -4,7 +4,7 @@
 
         <div class="input-group-btn" ngbDropdown>
             <button type="submit" class="btn btn-secondary">Search DSpace</button>
-            <button type="button" class="btn btn-secondary dropdown-toggle" data-toggle="dropdown"
+            <button type="button" class="btn btn-secondary dropdown-toggle dropdown-toggle-split" data-toggle="dropdown"
                     aria-haspopup="true" aria-expanded="false" id="searchDropdown" ngbDropdownToggle>
                 <span class="sr-only">Toggle Dropdown</span>
             </button>