Use node-web-streams

[canadaduane]

The client can be used in the browser or in a nodejs process. We use two separate mechanisms to read the stream in each case. A potentially more elegant solution was identified here:

// On nodejs, this requires "npm install node-fetch node-web-streams"
if (typeof window === 'undefined') {
    var fetch = require('node-fetch')
    var Headers = fetch.Headers
    var to_whatwg_stream = require('node-web-streams').toWebReadableStream
    module.exports = braid_fetch
}