Return instance ID's token directly, if needed (#27)

Instance ID always provides FCM of the default token via NSNotification. However, if you access the `FCMToken` property too early (i.e. immediately after calling `[FIRApp configure]`), it was returning nil. This ensures that the default FCM token is always returned, if it is available.

Author: rsattar

Firebase/Messaging/FIRMessaging.m

@@ -471,7 +471,12 @@ - (void)setAPNSToken:(NSData *)apnsToken type:(FIRMessagingAPNSTokenType)type {
 #pragma mark - FCM
 
 - (NSString *)FCMToken {
-  return self.defaultFcmToken;
+  NSString *token = self.defaultFcmToken;
+  if (!token) {
+    // We may not have received it from Instance ID yet (via NSNotification), so extract it directly
+    token = [self.instanceIDProxy token];
+  }
+  return token;
 }
 
 - (void)retrieveFCMTokenForSenderID:(nonnull NSString *)senderID