Fix risk of unfinished span when panic occurred in http handler

[njspok]

Last fixes 2b2d270#diff-3fde75a8887529bd00893a052634c273 require in handler h(sct.wrappedResponseWriter(), r) call WriteHeader. If not call WriteHeader defer mark span as error.

I use echo http server and dont call WriteHeader in my handlers - echo server alone make this call. In result all my spans in this middleware marked as errors.