Small patch to match a specific string in body(or header) for healthness

[ut0mt8]

Useful in some situation, you want to check something more precise than an http status. healthcheck.lua.patch.txt

[agentzh]

@ut0mt8 It's recommended to create github pull request to simplify code review and patch merging on our side. Thank you.

[agentzh]

@ut0mt8 BTW, receive("*a") may be VERY expensive when the response is huge. Don't do that.

[ut0mt8]

Well I see there is already a similar pull request, I think my version is more simple, I can just switch to receive(2048) or whatever reasonable value. Anyway a page which a healtcheck must be simple, Is not a good idea to check the regular homepage :)

[wtiancai]

i agree ut0mt8's opnion，the test page may has very simple content，just like ok