Modified the behavior of assignment to not modify the status code.

[gtnoble]

This is consistent with the behavior of bash/POSIX shell.
Before, it was impossible (to me) to get the status code when making an assignment from a command substitution.

Now you can have really neat C-like code like this:

if ( response=`{make-http-request $url} ) {
        echo $response
} else {
        echo 'an error occurred' >[1=2]
        echo $response
}