rdf generation from enums fails if the permissible value key includes a space

[cmungall]

this fails:

id: https://w3id.org/test
enums:
  test_enum:
    permissible_values:
      a b:

with

gen-rdf t.yaml
....
Exception: "https://w3id.org/test/a b" does not look like a valid URI, I cannot serialize this as N3/Turtle. Perhaps you wanted to urlencode it?

replacing the space with an underscore succeeds:

@prefix : <https://w3id.org/biolink/biolinkml/meta/> .
@prefix meta: <https://w3id.org/biolink/biolinkml/meta/> .
@prefix skos: <http://www.w3.org/2004/02/skos/core#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

<https://w3id.org/test/test> a meta:SchemaDefinition ;
    meta:default_prefix "https://w3id.org/test/" ;
    meta:default_range <https://w3id.org/test/string> ;
    meta:enums <https://w3id.org/test/test_enum> ;
    meta:generation_date "2021-02-23 16:21"^^xsd:dateTime ;
    meta:id <https://w3id.org/test> ;
    meta:metamodel_version "1.7.0" ;
    meta:source_file "t.yaml" ;
    meta:source_file_date "Tue Feb 23 16:21:36 2021"^^xsd:dateTime ;
    meta:source_file_size 81 .

<https://w3id.org/test/test_enum> skos:inScheme <https://w3id.org/test> ;
    meta:definition_uri <https://w3id.org/test/TestEnum> ;
    meta:permissible_values <https://w3id.org/test/a_b> .