Consider using a better `Accept` header when fetching modules

[kitsonk]

Currently the following header is sent when fetching modules:

Accept: */*

Which makes it very difficult for a server to do content negotiation. Either a header of:

Accept: application/javascript, application/typescript, text/javascript, text/typescript, text/jsx, text/tsx, application/json;q=0.9, text/plain;q=0.8, application/octet-stream;q=0.8

Or:

Accept: application/javascript, application/typescript, text/javascript, text/typescript, text/jsx, text/tsx, application/json;q=0.9, text/plain;q=0.8, application/octet-stream;q=0.8, */*;q=0.7

Would allow better negotiation of content for a web server.

Ref: denoland/deno#13287

[bnoordhuis]

#42 - I picked the first one.