Update tests to reflect the new HTTP dissector.
Update to reflect that BGP no longer prints a redundant length value.
Don't subtract the UDP header size from the length twice.
Fix length fields in UDP headers to be what they should be.

The value of the length field in a UDP header includes the length of the
header itself; the values in this capture didn't.  The length fields in
the IP headers and the RADIUS headers were correct and consistent with
each other, and the length fields in the UDP headers are now correct and
consistent with both of them.