iOS workaround: send a POST XHR request before _every_ sendBeacon call

this is more heavy-handed than we'd like and will pollute our bad-events stream; i'll tweak and find the exact times i need to make this request

Author: Scotty Jacobson

src/js/lib/beacon_ios_workaround.js

@@ -66,7 +66,7 @@
         // https://robertsahlin.com/analytics-beacon-transport-mechanism-gotchas/
         // https://bugs.webkit.org/show_bug.cgi?id=193508
         var xhr = new XMLHttpRequest();
-        xhr.open('POST', collectorUrl + '/com.snowplowanalytics/tp2', true);
+        xhr.open('POST', collectorUrl, true);
         xhr.setRequestHeader('Content-Type', 'text/plain');
         xhr.withCredentials = true;
         xhr.send('unlockBeacon');

src/js/out_queue.js

@@ -39,6 +39,7 @@
 		isString = require('lodash/isString'),
 		map = require('lodash/map'),
 		localStorageAccessible = require('./lib/detectors').localStorageAccessible,
+		beaconiOSWorkaround = require('./lib/beacon_ios_workaround'),
 		helpers = require('./lib/helpers'),
 		object = typeof exports !== 'undefined' ? exports : this; // For eventual node.js environment support
 
@@ -314,6 +315,7 @@
 						}
 						const blob = new Blob([encloseInPayloadDataEnvelope(attachStmToEvent(batch))], headers);
 						try {
+							beaconiOSWorkaround.unlockBeacon(configCollectorUrl);
 							beaconStatus = navigator.sendBeacon(configCollectorUrl, blob);
 						}
 						catch(error) {

src/js/tracker.js

@@ -45,7 +45,6 @@
 		links = require('./links'),
 		forms = require('./forms'),
 		errors = require('./errors'),
-		beaconiOSWorkaround = require('./lib/beacon_ios_workaround'),
 		requestQueue = require('./out_queue'),
 		coreConstructor = require('snowplow-tracker-core').trackerCore,
 		productionize = require('./guard').productionize,
@@ -2139,7 +2138,6 @@
 		 */
 		apiMethods.setCollectorUrl = function (rawUrl) {
 			configCollectorUrl = asCollectorUrl(rawUrl);
-			beaconiOSWorkaround.unlockBeacon(configCollectorUrl);
 		};
 
 		/**