SI-7322 Interpolator idents must be encoded

Otherwise, they are not found.

This matters for term names with a differential encoding.

Footnote, normally ident() encodes, but INTERPOLATIONID
is !isIdent, so that is not used here.  Maybe that would
be the better improvement.
Merge pull request #3424 from som-snytt/issue/7322

SI-7322 Interpolator idents must be encoded