Implement HasForeign instance

[afcady]

This is three patches, to servant-foreign, servant-multipart, and servant-js.

https://github.com/haskell-servant/servant-multipart/compare/master...afcady:multipart-foreign?expand=1

https://github.com/haskell-servant/servant/compare/master...afcady:multipart-foreign?expand=1

https://github.com/haskell-servant/servant-js/compare/master...afcady:multipart-foreign?expand=1

The patch to servant-foreign adds a Boolean field to the Req type specifying whether the request body is JSON (as is currently assumed). The patch to servant-multipart implements a HasForeign instance that sets this field to False. The patch to servant-js changes the output of vanillaJS when the flag is false, so that JSON.stringify() does not get called on the body and the content-type is not set to JSON.

This address haskell-servant/servant-multipart#5

[afcady]

Look OK now?

[alpmestan]

Ideally we would have something more general to support more content types, but let's roll with this for now as I don't have a brillant idea to make the supported content type list open-ended.