bugfix for Firefox for some ads that use document.write (prebid#3524)

* bugfix for Firefox for some ads that use document.write

* testing that the doc.open method is called.....

* added a sinon.spy() for the doc.open method

Author: jarnix

src/prebid.js

@@ -326,6 +326,7 @@ $$PREBID_GLOBAL$$.renderAd = function (doc, id) {
           const message = `Error trying to write ad. Ad render call ad id ${id} was prevented from writing to the main document.`;
           emitAdRenderFail(PREVENT_WRITING_ON_MAIN_DOCUMENT, message, bid);
         } else if (ad) {
+          doc.open('text/html', 'replace');
           doc.write(ad);
           doc.close();
           setRenderSize(doc, width, height);

test/spec/unit/pbjs_api_spec.js

@@ -990,6 +990,7 @@ describe('Unit: Prebid Module', function () {
 
     beforeEach(function () {
       doc = {
+        open: sinon.spy(),
         write: sinon.spy(),
         close: sinon.spy(),
         defaultView: {
@@ -1040,6 +1041,7 @@ describe('Unit: Prebid Module', function () {
       });
       adResponse.ad = "<script type='text/javascript' src='http://server.example.com/ad/ad.js'></script>";
       $$PREBID_GLOBAL$$.renderAd(doc, bidId);
+      assert.ok(doc.open, 'open method called');
       assert.ok(doc.write.calledWith(adResponse.ad), 'ad was written to doc');
       assert.ok(doc.close.called, 'close method called');
     });