Allow headers to be passed through to EventSource

Squashed commit of the following:

commit af6df1cfef7b92085e3dff4096bde8ee42b0341d
Author: Jordan Byron <[email protected]>
Date:   Tue Feb 20 21:13:06 2018 -0500

    Fix README

commit 69e229d21f6a527a23f994ed96fbf1e9fbca8cf1
Merge: 67682d6 fae9256
Author: Jordan Byron <[email protected]>
Date:   Tue Feb 20 21:08:21 2018 -0500

    Merge branch 'master' of https://github.com/dflourusso/react-native-event-source into dflourusso-master

commit fae9256
Author: Daniel Fernando Lourusso <[email protected]>
Date:   Thu Feb 15 08:25:26 2018 -0200

    Update README.md

commit b3ab975
Author: Daniel Fernando Lourusso <[email protected]>
Date:   Fri Jan 19 15:01:03 2018 -0200

    add options to accept headers

Author: jordanbyron

EventSource.js

@@ -6,7 +6,7 @@
 
 var reTrim = /^(\s|\u00A0)+|(\s|\u00A0)+$/g;
 
-var EventSource = function (url) {
+var EventSource = function (url, options) {
   var eventsource = this,
       interval = 500, // polling interval
       lastEventId = null,
@@ -18,6 +18,7 @@ var EventSource = function (url) {
   }
 
   this.URL = url;
+  this.OPTIONS = options;
   this.readyState = this.CONNECTING;
   this._pollTimer = null;
   this._xhr = null;
@@ -35,6 +36,11 @@ var EventSource = function (url) {
       // NOTE: IE7 and upwards support
       var xhr = new XMLHttpRequest();
       xhr.open('GET', eventsource.URL, true);
+      if (eventsource.OPTIONS && eventsource.OPTIONS.headers) {
+        Object.keys(eventsource.OPTIONS.headers).forEach(key => {
+          xhr.setRequestHeader(key, eventsource.OPTIONS.headers[key]);
+        });
+      }
       xhr.setRequestHeader('Accept', 'text/event-stream');
       xhr.setRequestHeader('Cache-Control', 'no-cache');
       // we must make use of this on the server side if we're working with Android - because they don't trigger

README.md

@@ -30,6 +30,13 @@ eventSource.addEventListener('message', (event) => {
 });
 ```
 
+You can also optionally pass a second argument when initializing `RNEventSource` to pass in header values:
+
+```js
+const options = { headers: { Authorization: 'Baerer ...' } };
+const eventSource = new RNEventSource('https://my-sse.com/stream', options);
+```
+
 Here is a full example that subscribes to a SSE stream and writes the results to `console.log`
 
 ```js

index.js

@@ -1,9 +1,10 @@
 import EventSource from './EventSource'
 
 class RNEventSource {
-  constructor(url) {
+  constructor(url, options) {
     this.url         = url;
-    this.eventSource = new EventSource(url);
+    this.options     = options;
+    this.eventSource = new EventSource(url, options);
     this.listeners   = [];
   }
   addEventListener (type, listener) {