Fix breadcrumbs to discard unusable metadata instead of cancelling call

* Replace invalid metadata with empty object
* Allow any object type as breadcrumb metadata

Fixes #36

lib/Bugsnag.js

@@ -104,9 +104,9 @@ export class Client {
     if (metadata.constructor === String) {
       metadata = {'message': metadata };
     }
-    if (!metadata instanceof Map) {
-      console.warn('Breadcrumb metadata is not a Map or String');
-      return;
+    if (!metadata instanceof Object) {
+      console.warn('Breadcrumb metadata is not an Object or String, discarding');
+      metadata = {};
     }
     let type = metadata['type'] || 'manual';
     delete metadata['type'];
@@ -244,7 +244,7 @@ const typedMap = function(map) {
   const output = {};
   for (const key in map) {
     const value = map[key];
-    if (value instanceof Map || value instanceof Object) {
+    if (value instanceof Object) {
       output[key] = {type: 'map', value: typedMap(value)};
     } else {
       const type = typeof value;