Fix the /tests URL in IE8

IE8 does not sent a text/html header, so tests don't load unless we allow */* requests to fetch the test file.

This is, unfortunately, complicated by a strange proxy requirement that `*/*` requests be passed through to the proxy as if they were JSON. That limits this fix to being pretty narrow, and I believe the wildcard route catcher will not work correctly in IE8.

I understand that history isn't supported in IE8 anyway, but still having the request treated like other browsers are treated seems more ideal.