multi-parameter 'value' (side-steps rack)

rack does not parse same-key parameters to an array so the
env["QUERY_STRING"] value (from rack) is parsed directly

https://github.com/rack/rack/pull/519

400 returned if value is not present (value+ required)
return multiple for equivalences in namespace
zero equivalences yield JSON null

previously this was an empty array
return nil if resource model type is not known