Fix ArrayStoreException in assignments to an element of an array of a single jso interface type.

Addresses issue 6448

Review at http://gwt-code-reviews.appspot.com/1470801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10414 8db76d5a-ed1c-0410-87a9-c151d255dfc7