Iterating over single value fails

Related to the difference in iterating over arrays (cf. #6), I also noticed that MappingWeaver fails when using the index wildcard [*] in a reference to a JSON value that is not an array. Again, I don't know what the RML spec would prescribe in such cases, but RMLViewer/RMLMapper nevertheless return the single JSON value instead.

Minimal example:

:view a rml:LogicalView ;
  rml:viewOn [
    # JSONPath root iterator over { "key": "value" }
  ] ;
  rml:field [ rml:fieldName "key" ; rml:reference "key[*]" ]
  .

:mapping a rml:TriplesMap ;
  rml:logicalSource :view ;
  rml:subjectMap [ rml:termType rml:BlankNode ] ;
  rml:predicateObjectMap [
    rml:predicate <urn:example:hasValue> ;
    rml:objectMap [ rml:reference "key" ]
  ] .

Start by reproducing issue #9 with the minimal RML mapping and JSON value shown in the report, then trace how MappingWeaver handles the [*] reference when the referenced value is not an array. Done means the single JSON value is returned consistently with the behavior described for RMLViewer/RMLMapper, with coverage for this example.