Prevent superfluous processing of already-processed form fields.
Since casper already handles fields with the same name (checkbox, radio)
in `getFieldValue`, we shouldn't process already-processed form fields
again in the main loop of `getFormValues`.
Bugfix: `getFormValues` choked on some radio inputs.
clientutils `getMultipleValues` choked on radio inputs where any radio
before the last one was chosen, mistakenly resetting the value to
`undefined`. Added spec to cover this case.
Merge pull request #375 from lukeasrodgers/bugfix_getFormValues

Bugfix get form values