Resolve with a POJO response

[satazor]

Instead of resolving with the WHATWG response, we could resolve with a POJO object. This would allow both js-ipfs and the service-worker-gateway to use the whole module. At the moment, there's some duplication going around.

[lidel]

+1
If we had POJOs, I could re-use entire thing in native handler of IPFS Companion as well.

Right now we need to duplicate code related to:

• mime-type sniffing
• setting HTTP Headers (Add Suborigin, Etag and X-Ipfs-Path headers #10 + feat(gateway): X-Ipfs-Path, Etag, Cache-Control, Suborigin js-ipfs#1537)
• handling directories, trailing slashes, returning redirects
• returning error codes such as 404 for non-existent unixfs child
• index.html lookup in sharded directories

Places where code is duplicated:

1. HTTP Gateway exposed by service-worker-gateway
2. HTTP Gateway exposed by js-ipfs
3. Native protocol handler in ipfs-companion