Shared secret rejections are not errors

[aronatkins]

This change has the shared secret filter no longer log an error and returns a cleaner error response.

options(`plumber.sharedSecret`="abcdefg")
p <- plumber::Plumber$new(file = NULL)
p$run()
# Running plumber API at http://127.0.0.1:8050
# Running swagger Docs at http://127.0.0.1:8050/__docs__/

curl http://127.0.0.1:8050/
# {"error": "Shared secret mismatch."}

Fixes #808

CC @blairj09

[CLAassistant]

All committers have signed the CLA.

[blairj09]

Thanks @aronatkins!

[meztez]

What should be the http status of these?

[schloerke]

What should be the http status of these?

From options_plumber:

plumber/R/options_plumber.R

Lines 38 to 40 in e846cb3

	#' \item{`plumber.sharedSecret`}{Shared secret used to filter incoming request.
	#' When `NULL`, secret is not validated. Otherwise, Plumber compares secret with http header
	#' `PLUMBER_SHARED_SECRET`. Failure to match results in http error 400. Defaults to `NULL`.}

[schloerke]

LGTM pending suggestion

(Windows devel is unrelated to failure)
(Please squash+merge when ready)

[schloerke]

Thank you for the tests @aronatkins!