StdLibAdapter doesn't correctly recognize panics from net/http when running a http server

When running a http server using the stdlib's net/http (the idiomatic way to do this in Go) and when using `stdlog.SetOutput(log.NewStdlibAdapter(logger))`, any panics recovered from by net/http and then printed out will be mangled. Only the top line will be shown in the msg field. Additionally, the file field is wrong.