feat(active) support map headers

[mayocream]

Summary

• change active.headers values to one or more lists of values indexed by header name.
• deprecate array headers.
• escape characters in HTTP headers (See https://www.rfc-editor.org/rfc/rfc7230#page-9)

[flrgh]

👍 this looks good to me, though it might be wise to get a second review from somebody who has done more work in this repo (@locao or @Tieske maybe?)

[mayocream]

And a new release is needed for Kong I think?

• feat(balancer) upstream healthcheck headers kong#8255