[PATCH] Fix bang flag in ${(!)ref::=var} substitutions

The documentation states that with the substitution flag (!) the reference
itself is examined, not its referent, but this fails in ${(!)ref::=var}
substitutions:

% typeset var=foo
% typeset -n ref=var
% echo ${(!)ref::=bar}
bar
% typeset -p var ref
typeset var=bar
typeset -n ref=var

The result of the last command should be the following:

typeset var=foo
typeset -n ref=bar

- Fix bang flag in ${(!)ref::=var} substitutions
<https://github.com/paltherr/zsh/compare/fix-bang-flag-in-substitutions-start...paltherr:zsh:fix-bang-flag-in-substitutions>

Philippe