v9: Cleanup http interface

I run into SyntaxError: Unexpected token < in JSON at position 0

It seems that it may be caused by the same issue as #384 - where http error page was parsed as json.

I want to check is it actually happening but right now I have no idea how to get response code of a failing fetch.

// SyntaxError: Unexpected token < in JSON at position 0
wtf.fetch('https://wiki.openstreetmap.org/wiki/Tag:highway%3Dmotorway').then((doc) => {
    //console.log(doc.sentences(0).text())
    //console.log(doc.infobox())
    //console.log(doc.infobox().json())
})

// works
wtf.fetch('Poland').then((doc) => {
  console.log(doc.sentences(0).text())
  console.log(doc.infobox())
  console.log(doc.infobox().json())
})

/*
// from readme sample
// fails to parse infobox
wtf.fetch('https://muppet.fandom.com/wiki/Miss_Piggy').then((doc) => {
    console.log(doc.sentences(0).text())
    console.log(doc.infobox())
    if(doc.infobox() != undefined) {
        console.log(doc.infobox().json())
    }
})

*/

[wvanderp]

I was already looking at this for a bit but christmas got in the way. I already wrote out my usual wall of text and will post it tomorrow.

[spencermountain]

in addition to allowing users to set custom headers, per @Vacilando , we should set this as a default header - https://www.mediawiki.org/wiki/API:Etiquette#Request_limit recommends to set Accept-Encoding: gzip