Bad Content-Length returns 500 response as of Go 1.2

This was a regression found when compiling and testing the router
under go 1.2 compared to 1.1.2 (currently built against version).
Since go 1.2 the behaviour for how bad Content-Length headers are
handled has changed from providing a (private) error value that we
could match against through to directly returning an
`io.ErrUnexpectedEOF` error.

Marking this test as pending as it may eventually start working again
and somebody should look at it again when that happens.

Updating our second test case (Content-Length bigger than request
body) to match the new behaviour in Go 1.2.