Preserve any passed :filter-map
Preserve any passed :filter-map

Pushing for a user. We still need tests and docs around this,
specifically wrt to preserving insertion order and whether it makes
sense for WebSocketHelpyHelpertonFilter to always be last.

immutant/immutant,kbaribeau/immutant,immutant/immutant,immutant/immutant,kbaribeau/immutant,kbaribeau/immutant,immutant/immutant