Add formUrl body type

[elbywan]

Sets the content encoding header to "application/x-www-form-urlencoded" and sets the body.

// With a string
wretch("myurl").formUrl("say=Hi&to=Mom").post()

// Provide a simple way to serialize objects (no funky stuff)
wretch("myurl").formUrl({ say: "Hi", to: "Mom"}).post()

[elbywan]

const form = { a: 1, b: { c: 2 }}
const alreadyEncodedForm = "a=1&b=%7B%22c%22%3A2%7D"

wretch("...").formUrl(form).post()
wretch("...").formUrl(alreadyEncodedForm).post()