data must be imported as optional by Swift. (#43)

* data must be imported as optional by Swift.

* Explicitly pass nil for clarity when invoking onFailure.

Author: lk23lekkladf2

Iterable-iOS-SDK/IterableAPI.h

@@ -21,7 +21,7 @@ typedef void (^OnSuccessHandler)(NSDictionary *data);
 /**
  The prototype for the completion handler block that gets called when an Iterable call fails
  */
-typedef void (^OnFailureHandler)(NSString *reason, NSData *data);
+typedef void (^OnFailureHandler)(NSString *reason, NSData *_Nullable data);
 
 /**
  Enum representing push platform; apple push notification service, production vs sandbox

Iterable-iOS-SDK/IterableAPI.m

@@ -244,7 +244,7 @@ - (void)sendRequest:(NSURLRequest *)request onSuccess:(void (^)(NSDictionary *))
             if (onFailure != nil) onFailure(@"No data received", data);
         } else if (error != nil) {
             NSString *reason = [NSString stringWithFormat:@"%@", error];
-            if (onFailure != nil) onFailure(reason, data);
+            if (onFailure != nil) onFailure(reason, nil);
         }
     }];
     [task resume];