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.

Form encoding an Object doesn't work for keys that occur several times in one request body, like `issues[]=12&issues[]=42`. Use FormData as the request body to POST form fields instead.

/cc @annevk on spec compliance.