tests: move jsonClone() and roundTrip() into testhelpers.js
test/misc: add test for non-combined headers

See http://tools.ietf.org/html/rfc3261#section-7.3.1
Take non-combined headers into account when building/serializing JSON

Their values go into a single array, but are serialized into separate fields.
grammar: Fix challenge rule by making it more like the credentials rule

Adds the _digest_challenge rule, modeled after digest_response.
grammar: domain/qop_options rules include quotes when serialized

pass the non-combined headers test added in b328a8f67dc0a87fd76f4a55d0974acd4100648f