Fix autocompletions for ZSH

`wp core [tab]` results in _wp_complete:local:4: not valid in this context: download

Wrapping with quotes seems to fix the immediate issue. 

Extra eyes to make sure nothing else needs to be changed as well as verification this still works in bash, etc is requested. 

More info on quotes in ZSH here: http://unix.stackexchange.com/questions/68694/when-is-double-quoting-necessary/68748#68748