Remove form encoded object body.

This was based on a misreading of the URLSearchParams portion of
section 5.2 of https://fetch.spec.whatwg.org/#body-mixin.

Use FormData as the request body to POST form fields instead.
Fix form post content type test.
Remove old body property references.

The body must be consumed through the `text`, `json`, `formData`, or
`blob` functions.
Remove object form encoding test.
Merge pull request #30 from github/remove-form-encode

Remove form encoded object body.