Dedupe conflicting variable names for forms with multi-select fields

There were two variables that were re-declared in the ```if (element.type === 'select-multiple')``` block:  ```i``` and ```options```

I renamed them to ```j``` and ```selectOptions```, respectively, to avoid conflicts.

Otherwise you start getting undesirable behavior when you're serializing a form with multi-selects. Namely the outer ```i``` gets incremented in the inner loop, causing the serialized form to "skip" fields.