CREDENTIAL: Drop 'send()', add 'toFormData()'.

Based on the discussion in #241, moving to a 'fetch()'-based model seems
like the right thing to do for this API. I'm a bit concerned at the loss of granular
control over the options passed into 'fetch()' (e.g. no redirects, same-origin
endpoint, etc). Perhaps we can extend 'FormData' a bit more?

Author: mikewest