Fix compilation of SpEL Indexer nodes involving map references

There is special handling for SpEL expressions involving a map
and an unquoted string literal key (e.g. mymap[key1]). SpEL does
not require key1 to be quoted. This special handling which is done
in Indexer getValueRef() was not being also done in the Indexer
generateCode() method that compiles the expression. Also fixed
a problem where the key was not being compiled in a new
sub scope. Without the new scope the key expression was failing
to reload the relevant context object when it needed it.

Issue: SPR-12045