clean up invalid_multibyte test

This cleans up the invalid_multibyte test to make it more clear
what it is trying to do, which is send an invalid utf8 sequence as
an image name (it should fail with a 400, it currently passes with
a 202).

Change-Id: I58ad059a138ae96201bbf50df0c22bf259ae9716
Related-Bug: #1006725
Merge "clean up invalid_multibyte test"