change to how code inference works when value set includes a single c…

[grahamegrieve]

…ode system

[grahamegrieve]

what's going on here is that my server used to assume that if it was asked to infer a code system, and the value set only included codes from one code system, that the code was in the code system. Then if the code wasn't actually in the code system, it would say it was an invalid code

but in fact, that's not quite the case - if the code isn't in the value set, then we can't assume it's intended to be. And it turns out that's what's driving this case: hapifhir/org.hl7.fhir.core#1564