wasm: fix standalone/multipart body conversion to JsValue

[nwolber]

This is an attempt to fix the conflicting conversion code requried to turn a Body into a JsValue depending if the body is standalone or if it is part of a form (aka multipart/form-data).

The idea is to introduce a new variant to the crate::wasm::body::Inner enum that holds the actual content of a body. The new variant allows Body.to_js_value to have dedicated codepaths for standalone bodies and form parts. The new variant is contructed by the new Body.into_part method. That method is called by the crate::wasm::multipart::Part::new constructor and turns an Inner::Body into the new Inner::MultipartPart variant.

The drawback is that the new variant leads to some code duplication in the Body methods where it mostly shares code with the Inner::Bytes variant.

For clarity the PR renames the Inner::Multipart variant to Inner::MultipartForm. It also adds new test cases to cover the serialization of different bodies either standalone or multipart.

Thanks to @crapStone for pointing at the Inner enum in #1358 and @skystar-p for the original mutlipart conversion code in #1341.

[seanmonstar]

Would you be able to add a comment to the variants to help us remember what the difference is between the two? (It's still not clear to me XD).

[nwolber]

Sure, I give it a try. Tell me if it is clearer now or where I should go more into detail.

[SionoiS]

Would this fix #1141 ?

[ghost]

@seanmonstar This is a critical fix for sending multipart requests via WASM and review of this code should be prioritized. I can confirm through my testing that multipart requests work correctly after applying this patch.