Add WriteHeader call to Flush

[beorn7]

Flush is another of the methods that will call WriteHeader if it
hasn't happened yet. Since we want to call observeWriteHeader (if
set), we need to do the WriteHeader call already here, similar to what
we have done in Write and ReadFrom.

This commit also adds comments explaining the above to not tempt
developers to remove the WriteHeader call.

Signed-off-by: beorn7 beorn@grafana.com

[matthiasr]

How does the ResponseWriter know that we already called WriteHeader? Could we rely on the same mechanism instead of implementing our own (wroteHeader)?

[beorn7]

It really depends on the implementation. The implementation I looked at in the standard library (https://golang.org/src/net/http/server.go) do the same thing, i.e. they have an unexported tracking variable wroteHeader. Since it is unexported, the delegator we are implementing here cannot access it and we have to do our own tracking.