Updates for Web Worker

[SinanMtl]

Problem

When I was imported graphql.js on Web Worker, I saw these error messages on console:

Uncaught TypeError: Cannot set property 'graphql' of undefined
  at Object.<anonymous> (graphql.js:524)

ReferenceError: headers is not defined
  at caller (graphql.js:321)

ReferenceError: __doRequest is not defined
  at __request (graphql.js:109)

Solutions

self added for callback context.

;(function (root, factory) {
  ...
}(this || self, function () {
  return GraphQLClient
}))

Added var keyword to headers variable.

GraphQLClient.prototype.createSenderFunction = function (debug) {
  ...
  var headers = ...
  ...
}

__doRequest has been changed like below:

var __doRequest
  ....

if (typeof XMLHttpRequest !== 'undefined') {
  __doRequest = function () {
    ....
  }
} else if (typeof require === 'function') {
  __doRequest = function () {
    ....
  }
}