resolve #11 - fix multipart test content length value

change this to a \d+ check rather than an explicit check since this
may change. this appears to be caused by a recent change in the core
Mojolicious code: 34c7b4f2d16244c641c27bee54112dff18e94f38 but the
commit message doesn't tell us *why* 2 extra bytes were added to the
content_length value (it seems to be due to an extra \x0d\x0a at the
end of the boundary definition)