Fix dropdown search sizing

When Select2 is not used with a design that universally sets
`box-sizing: border-box` to elements, the search box previously
would take up more space than expected. This fixed the issue by
adding the `box-sizing` rule to the search box.

This closes select2#2978.

Author: kevin-brown

dist/css/select2.css

@@ -83,7 +83,8 @@
   padding: 4px; }
   .select2-search--dropdown .select2-search__field {
     padding: 4px;
-    width: 100%; }
+    width: 100%;
+    box-sizing: border-box; }
   .select2-search--dropdown.select2-search--hide {
     display: none; }