bundler: make it play nice with functions that call wrapped commands (#8271)

When another function calls one of the bundler plugin's wrapper functions, the
command to run gets passed as an array instead of a space-separated string. That
works fine when the arguments are expanded alone, like `bundle exec $@`, but
something like `./bin/$@` will expand to something like `./bin/rake
./bin/--silent ./bin/--tasks`, which of course will explode. This was causing a
nasty interaction with the rake-fast plugin, and I'd be shocked if it wasn't
causing other problems.

The fix is to explicitly turn off the `RC_EXPAND_PARAM` option for that expansion.
See http://zsh.sourceforge.net/Doc/Release/Expansion.html#Parameter-Expansion
for more details.