[JavaScript] Support optional parameters with object syntax

[xhh]

See #2027

Code sample of optional parameters:

// specify both "name" and "status"
petApi.updatePetWithForm(1, {name: 'new name', status: 'pending'}, function (err, data, resp) { });
// specify only "status"
petApi.updatePetWithForm(1, {status: 'sold'}, function (err, data, resp) { });

[wing328]

@delenius please take a look when you've time to see if that's what you've in your mind.

[delenius]

Looks good to me.
@xhh , would you mind also updating the javascript-promise version of the petstore example (and run the tests for that)? i.e. bin/javascript-promise-petstore.sh.