fix type of completion handler to avoid use of IUO

Author: ishkawa

APIKit/APIKit.swift

@@ -40,9 +40,9 @@ private extension NSURLSessionDataTask {
         }
     }
 
-    private var completionHandler: ((NSData!, NSURLResponse!, NSError!) -> Void)? {
+    private var completionHandler: ((NSData?, NSURLResponse?, NSError?) -> Void)? {
         get {
-            return (objc_getAssociatedObject(self, &dataTaskCompletionHandlerKey) as? Box<(NSData!, NSURLResponse!, NSError!) -> Void>)?.unbox
+            return (objc_getAssociatedObject(self, &dataTaskCompletionHandlerKey) as? Box<(NSData?, NSURLResponse?, NSError?) -> Void>)?.unbox
         }
 
         set {
@@ -167,18 +167,24 @@ public class API: NSObject, NSURLSessionDelegate, NSURLSessionDataDelegate {
                     dispatch_async(mainQueue, { handler(.Failure(Box(error))) })
                     return
                 }
-
-                let mappedResponse: Result<T.Response, NSError> = self.responseBodyParser().parseData(data).flatMap { rawResponse in
-                    if let response = request.responseFromObject(rawResponse) {
-                        return success(response)
-                    } else {
-                        let userInfo = [NSLocalizedDescriptionKey: "failed to create model object from raw object."]
-                        let error = NSError(domain: APIKitErrorDomain, code: 0, userInfo: userInfo)
-                        return failure(error)
+                
+                if let data = data {
+                    let mappedResponse: Result<T.Response, NSError> = self.responseBodyParser().parseData(data).flatMap { rawResponse in
+                        if let response = request.responseFromObject(rawResponse) {
+                            return success(response)
+                        } else {
+                            let userInfo = [NSLocalizedDescriptionKey: "failed to create model object from raw object."]
+                            let error = NSError(domain: APIKitErrorDomain, code: 0, userInfo: userInfo)
+                            return failure(error)
+                        }
+                        
                     }
-
+                    dispatch_async(mainQueue, { handler(mappedResponse) })
+                } else {
+                    let userInfo = [NSLocalizedDescriptionKey: "unable to get response body despite NSURLSession raised no error."]
+                    let error = NSError(domain: APIKitErrorDomain, code: 0, userInfo: userInfo)
+                    dispatch_async(mainQueue, { handler(.Failure(Box(error))) })
                 }
-                dispatch_async(mainQueue, { handler(mappedResponse) })
             }
 
             task.resume()