Add `ToRequestBody RequestBody` instance

[pete-murphy]

I was hoping to use fetch in Node, and to pass a Buffer as request body, but this is not currently possible.

main :: Effect Unit
main = do
  buffer <- Buffer.fromString "Hello, world" UTF8
  Aff.launchAff_ do
    void (Fetch.fetchBody "http://example.com" { body: buffer })

This fails to compile because there's no ToRequestBody instance for Buffer.

I opened an issue in web-fetch purescript-web/purescript-web-fetch#15 about adding RequestBody.fromBuffer (which would enable adding a ToRequestBody Buffer instance here) but that's not the right approach as it would incur a dependency on node-buffer and make both the "core" web-fetch and this fetch not portable between environments.

I think maybe a better solution would be to have instance ToRequestBody RequestBody in this library, and then a hypothetical node-fetch could wrap this fetch implementation and have its own RequestBody module with Node-specific conversion functions like fromBuffer :: Buffer -> RequestBody:

main :: Effect Unit
main = do
  buffer <- Buffer.fromString "Hello, world" UTF8
  Aff.launchAff_ do
    void (Fetch.fetchBody "http://example.com" { body: RequestBody.fromBuffer buffer })

Let me know if that makes sense, or if there are reasons for omitting the instance ToRequestBody RequestBody that I'm over-looking. Thanks!

[sigma-andex]

fetch depends on fetch-core (a fork of web-fetch). We planned to move that to purescript-contrib and deprecated web-fetch.