Revert "Subtly change semantics of R:I.NORMALIZE_ENCODING"

This reverts commit 22ea9d9.

This was done to accommodate PR #4231, but I think we need to get
back to the drawing board for that PR, as the reverted commit broke
the error reporting on: Blob.new(65,66,67).decode("iso-8859-2")

src/core.c/Rakudo/Internals.pm6

@@ -350,7 +350,7 @@ my class Rakudo::Internals {
             nqp::atkey($encodings,encoding),
             nqp::ifnull(
               nqp::atkey($encodings,nqp::lc(encoding)),
-              Nil
+              nqp::lc(encoding)
             )
           ),
           'utf8'