Fix type valueset binding to extensible

[costateixeira]

The reason HL7 Intl left it as preferred is because there were 2 codes before, and their meaning obviously does not map 1:1 to the 3 codes we'd want or other sets of codes. Since we want to enforce these codes to be used, like in the rest of the spec, this should be extensible.

Note: preapplied in the release candidate.

[bdc-ehealth]

@costateixeira Taking into account the attitude of Isabelle Pollet, I'd prefer to present this change to a WG. @costateixeira Don't bother this is already released. It can be closed.