MINOR: payload: allow the payload sample fetches to retrieve arbitrary lengths

When using req.payload and res.payload to look up for specific content at an
arbitrary location, we're often facing the problem of not knowing the input
buffer length. If the length argument is larger than the buffer length, the
function did not match, and if they're smaller, there is a risk of not getting
the expected content. This is especially true when looking for data in SOAP
requests.

So let's make some provisions for scanning the whole buffer by specifying a
length of 0 bytes. This greatly simplifies the processing of random-sized
input data.