WFE: reject legacy JWS requests in -strict (#246)
WFE: reject legacy JWS requests in -strict (#246)

* wfe: reject out-of-spec chall POST in strict.

Challenge POST JWS bodies should be exactly `{}` per RFC 8555 Section
7.5.1. In `-strict` reject anything else.

* wfe: reject JWS JSON with non-empty "resource".

The "resource" field of a JSON JWS body is a legacy artifact of ACME v1
and should not be sent in RFC 8555/ACME v2 requests. Reject these
requests in `-strict` mode.

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