Update spoofed activity test

Turns out we already had a test for activities spoofed via upload due
to an exploit several years. Back then *oma did not verify content-type
at all and doing so was the only adopted countermeasure.
Even the added test sample though suffered from a mismatching id, yet
nobody seems to have thought it a good idea to tighten id checks, huh

Since we will add stricter id checks later, make id and URL match
and also add a testcase for no content type at all. The new section
will be expanded in subsequent commits.