Allow setHeaders while using dev mode

[kristoferbaxter]

I have a use-case where even in dev mode I need specific headers to be set, as a result I'm using prod mode but restarting node when FS changes are detected.

Would adding setHeaders into dev mode be a huge burden?

[lukeed]

Not a burden at all~! Will add it now, makes sense to have.

[kristoferbaxter]

Thanks!