Use UTF-8 coding rather than the default ISO-8859-1 to get values for keys in MultipartRequestResolver

[taojing2002]

In MultipartRequestResolver.resolveMultipart method, we get values for keys:

String name = item.getFieldName();
String value = item.getString();

This will use the default ISO-8859-1 to encode the value, which can't not handle special characters. Metacat has ticket about the Chinese search issue. That issue roots from here.

We should use:

String name = item.getFieldName();
String value = item.getString(StandardCharsets.UTF_8.name());