FormData refactor

[mbyczkowski]

This brings FormData back.

You can just do (it's available globally in the runtime):

const fd = new FormData

or get it straight from the Response:

const fd = response.formData()

Implements #16

[coveralls]

Pull Request Test Coverage Report for Build 329

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• 95 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.8%) to 65.486%

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Files with Coverage Reduction	New Missed Lines	%
src/bridge/fetch.ts	2	91.89%
src/server.ts	2	67.35%
src/context.ts	2	83.65%
src/utils/buffer.ts	3	75.0%
src/bridge/heap.ts	5	37.5%
src/v8env.ts	12	71.43%
src/bridge/fly/cache.ts	16	69.44%
src/bridge/formdata.ts	22	8.82%
src/utils/build.ts	31	20.0%

Totals
Change from base Build 322:	-0.8%
Covered Lines:	721
Relevant Lines:	1043

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💛 - Coveralls

[jeromegn]

This is probably a good first step, but I don't think it's following the spec (not that the previous implementation was especially spec-compliant, but we may as well do it right this time around).

Check out the FormData and URLSearchParams bits of https://fetch.spec.whatwg.org/#body-mixin

Your current implementation looks a lot like what the fetch spec prescribes for URLSearchParams. I'm just learning this now too btw :) Seems like our currently included whatwg-url class handles URLSearchParams right, we just need to detect it as the bodySource and set the right headers automatically.

That's something we have not being doing quite right to be honest. We need to start looking in the fetch spec more to provide a better, more predictable, implementation.

If you want to convert your current PR to do the simple, small URLSearchParams fetch body switch case, that's probably more encouraging and shippable in a short amount of time :)

URLSearchParams

• Set action to an action that runs the application/x-www-form-urlencoded serializer with object’s list.
• Set Content-Type to application/x-www-form-urlencoded;charset=UTF-8.
• Set source to object.

[jeromegn]

Never mind my previous comment, the FormData is on spec, except for how it's serialized in the body.

I believe the tests should use a Request and match a request precisely with the right serialization.

[jeromegn]

I don't think an example with a Response is super relevant. I think FormData will be much more useful in requests.