Add ability to check existence of multiple keys

[bvisness]

exists can now accept an array of keys to check for existence.

[shadowhand]

What would be the purpose of this? As compared to:

$subset = expect($array, $keys);

Generally when working with multiple keys the ultimate need is to capture a known hash of values and pass that on somewhere else. Can you provide an example of how you would use this?

[bvisness]

@shadowhand I was making a REST endpoint and wanted to check that all the required fields existed in $input. In that case expect didn't really make sense because I don't want any default values to be used.

[bvisness]

(I ended up using array_diff_key anyway so I could show the user what fields they were missing...but that's beside the point. 😛 )