Fix Audit-ID header key
Fix Audit-ID header key

Now http header key "Audit-ID" doesn't have effect, because golang
automaticly transforms "Audit-ID" into "Audit-Id". This change use
http.Header.Get() function to canonicalize "Audit-ID" to "Audit-Id".

Kubernetes-commit: f21bc7bb9a82378e8b24f72c66dfd23bc8113f20

caesarxuchao/apiserver,kubernetes/apiserver,kubernetes/apiserver,caesarxuchao/apiserver,caesarxuchao/apiserver