fix: use try/catch when setting xhr.responseType

Some XHR implementations (like Firefox WebWorker, react-native and some Android 4.x versions) throw
an exception when setting xhr.responseType = 'arraybuffer' when xhr.readyState === 2 (which is
perfectly valid, spec-wise).

That commit fixes that behaviour by reverting some of the changes from
#562 for those implementations.

Closes #590

lib/transports/polling-xhr.js

@@ -220,6 +220,10 @@ Request.prototype.create = function () {
       xhr.setRequestHeader('Accept', '*/*');
     } catch (e) {}
 
+    if (this.supportsBinary) {
+      xhr.responseType = 'arraybuffer';
+    }
+
     // ie6 check
     if ('withCredentials' in xhr) {
       xhr.withCredentials = true;
@@ -239,13 +243,12 @@ Request.prototype.create = function () {
     } else {
       xhr.onreadystatechange = function () {
         if (xhr.readyState === 2) {
-          var contentType;
           try {
-            contentType = xhr.getResponseHeader('Content-Type');
+            var contentType = xhr.getResponseHeader('Content-Type');
+            if (contentType !== 'application/octet-stream') {
+              xhr.responseType = 'text';
+            }
           } catch (e) {}
-          if (contentType === 'application/octet-stream') {
-            xhr.responseType = 'arraybuffer';
-          }
         }
         if (4 !== xhr.readyState) return;
         if (200 === xhr.status || 1223 === xhr.status) {
@@ -355,7 +358,11 @@ Request.prototype.onLoad = function () {
       contentType = this.xhr.getResponseHeader('Content-Type');
     } catch (e) {}
     if (contentType === 'application/octet-stream') {
-      data = this.xhr.response || this.xhr.responseText;
+      if (this.xhr.responseType === 'arraybuffer') {
+        data = this.xhr.response || this.xhr.responseText;
+      } else {
+        data = String.fromCharCode.apply(null, new Uint8Array(this.xhr.response));
+      }
     } else {
       data = this.xhr.responseText;
     }