Can't use mutable header APIs after sent.

[tj]

IMO you should be able to use getHeader() after for reporting etc

[ncb000gt]

+1 - @ry also said to mention @creationix in the ticket.

[focusaurus]

Agreed. I have also hit this issue. in http.js OutgoingMessage.getHeader will throw an error if _header is already set, but since this is a read-only API, it's not appropriate. I ran this by ryah in IRC and he agreed this error is probably not needed here. He asked me to mention @creationix in the github issue. The fix is just to delete the clause that raises the error.

[creationix]

I'm find with removing the check since it's read-only. I was just being conservative in my patch and allowing as little new functionality as possible.