[views] fix ecsvexport selector introduced by e48e5a597ccc Fix the view and make web/test/test_views.py pass again.

Add custom checker for Password values We override the default converter to pass Binary values through, but don't do anything about the checker. This worked previously because yams allowed StringIO instances, although its converter didn't do the right thing for them. Fixing this in yams requires that we properly register a checker.