Merge pull request #1518 from OneSignal/fix/NSInvalidArgument-Exception

Fix: Prevent crash with nil HTTPResponse headers

Author: jennantilla

iOS_SDK/OneSignalSDK/OneSignalCore/Source/API/OneSignalClient.m

@@ -197,7 +197,7 @@ - (void)handleJSONNSURLResponse:(NSURLResponse*)response data:(NSData*)data erro
 
     NSHTTPURLResponse* HTTPResponse = (NSHTTPURLResponse*)response;
     NSInteger statusCode = [HTTPResponse statusCode];
-    NSDictionary *headers = [HTTPResponse allHeaderFields];
+    NSDictionary *headers = [HTTPResponse allHeaderFields] ?: @{};
     NSError* jsonError = nil;
     NSMutableDictionary* innerJson;